Page inset on master doesn't apply to document page

Here’s my primary text frame test—drag the B-master onto page 7 and top inset is applied:

https://shared-assets.adobe.com/link/1ed870d3-8d6a-4437-511e-aa7071b00567

There is an alternative to make Page sink so you do ot need a new master. In the Chapter paragraph style (if chapter is the paragraph style for the first text after page sink) add a Paragraph rule Above. Offset that paragraph rule to the distance you want from the first baseline from the frame edge and check "Keep in Frame", set the rule to be no colour, no weight.
(Note screen shot just the paragraph rule setting nit applied to a Paragraph style)

The problem is that when you thread text in a master page it adds content. You can sometimes get an OK result if the master page with sink is based on the first master page and you have only overridden  the height of the frame. An alternative would be to create your sink with a textwrap  (on your second master) so you do not need to modify the actual Primary text frame. 

Sorry I bypassed your initial question and went to a solution that I know works for page sinks. 

I had no knowledge of your file setup, or how many master pages you have, and thought the way I do it works, so I'll drop that in there and see if resolves your issue - and of course it's a public forum so it might help someone years down the line.

I very rarely have more than 2 or 3 master pages, and I've designed books up to 3,200 pages.

Anyway - typically if a Master Page is not being honored on the page it's because the page masters have been overridden. 

There's a lot of replies in this thread and I'm not sure which one was effective for you.

But I can only contribute and continue with what I know works. 

As you say -  you're experienced and have not come across this before - I'll skip the techniques to replace master pages, I assume you've already come full circle on this. For anyone else with this issue there is a help file for Master Pages:

https://helpx.adobe.com/indesign/using/master-pages.html

I've had books all of a sudden stop working - I don't always use the book panel either. A lot of the time it is overkill. The only time I find it really useful is using Chapter numbers and where an author is shifting chapters around until final output. Otherwise, I typically keep the book in one file. if it goes over a certain amount of pages I might split it into a Book file to save the computer. 

Anyway - back to your issue and based on my experience of what goes wrong.

1. Corrupt pages (actual page is corrupted)

2. Text frame is corrupted

3. Styles are corrupted

4. Table is corrupted

5. Images are corrupt

I keep thinking corrupt - because what you describe is abnormal - and you're an experienced InDesign user familiar with Master pages. 

Typically a round trip to IDML irons out 9/10 corruption issues and works the first time.

For people new to InDesign and just finding this thread:

File>Export

Choose IDML

Open your new IDML file in InDesign

Resave as an InDesign file with a different file name (version number is pretty good to keep things clean)

This can remove minor corruption from files, like wonky styles etc.

But sometimes this doesn't work. I've had this before, the IDML roundtrip did nothing and the problem persisted.

Another approach I have found works well is to create a new document and use the Pages Panel to Move your pages to the new document

- I'm going to give the steps for this - as someone else might find this thread and find it useful.

Start by creating a new document - using the exact same margins and page size as your original document. 

You don't really need to worry about inserting master pages - this will happen for you when you move the pages over.

You can have just 1 page with the same margins - nothing else

Next, open your document that has issues (if already open the go-to that document tab)

In the Pages panel (Window>Pages)

Use the sub menu on the top right

Move Pages

Select All Pages

And then also select your new document as the Move To:

There is a caveat to this

If you have used Styles - you might have inadvertently set some styles to be Based on Basic Styles - and if you've changed Basic Styles you might get weird results.

There's more on this here

https://creativepro.com/beware-basic-styles/

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I appreciate this might not work specifically for your instance. And I'm sorry if it's not the answer you're looking for. But it's very hard to know what's going on - so I can only surmise and try to give you my experience and techniques to fix things that have gone wonky.

If it doesn't help you then perhaps someone else will find this public forum post in the future and it might help them.

I'm not having the same issue.

I just created a new 8-page document, with a Master Page A for the main pages.

I then created a new Master B BASED ON Master Page A, where I edited the text frame to incorporate the sink.

It works as intended throughout the document.

Hmm.

Hi @casinclaire , to share the text flow between masters, you need to use Primary Text Frames. On the initial document setup you can check Primary Text Frame and primary text frames will be set to the A-Master‘s margins. 

Primary frames are automatically overriden on the pages and can be shared between masters. There can only be one primary text frame per master page. You can right click a master text frame to toggle Primary Text Frame on or off—the Primary Text Frame icon has an arrow. Here I’ve copied my A-Master, set the top inset of the B-Master, and made sure both frames are set as Primary Text Frames:

Now dragging the B-Master onto the chapter opener creates the desired inset

Typically for books I have 2 master pages.

1st master page is for page 1 - with margins set at the sink level

2nd master page is for the rest of the text. 

Then all chapters are combined as separate files in the Book Panel.