When clicking on a thumbnail in the grid view, I get a large grey image with a ? in the middle. After a while, I get returned back to the grid view. I've just noticed that when you try and view an image with a person on it, moving the cursor will actually identify the person/face, but still no image!! I've tried clearing the cache and reloading the thumbnail without luck
Wee, in that moment your catalog was initially created, Organizer learned the locations of the images on your hard drive (wherever they were at that moment - wherever you had them, no matter where that was). Organizer then works off that location info in the future. Example, you mention clicking a thumbnail in the grid and seeing only a question mark. You're likely seeing a question mark because Organizer went to the memorized location on your hard drive, expecting to find that image - ie: where it was when the catalog was first created, but it was no longer there. Why?
The likeliest explanation is that you either renamed, moved, or deleted it since the time your catalog was originally created. Many users new to Organizer incorrectly believe that their images are stored inside the organizer but that isn't the case. The thumbnails you see in the grid are only visual references to images that exist somewhere else on you hard drive. A properly cared for Organizer will always know where to find them, even after moving or renaming them, and it will know not to look for deleted ones too, but there's only one way to do any of these things after a working catalog (like yours) has been created.
Your next question probably has to do with how to fix your current catalog and best ways to rename, move, delete stuff from your catalog so things run smoothly again. Since there are better qualified people in here to assist with those things, I'll step back now so that can happen. Don't be discouraged.
Thank you for your quick response - I had thought of this, but have been using elements for the last 10 years and had actually hit that problem before.
I received an email from Adobe support just after posting and have spent the last hour with them and have a resolution of sorts...
It looks like the last update does not like apple's 5k screen - if you reduce the size of the window, the preview displays, make the window bigger, it just shows grey - its a fine art getting it to a size that displays!!
So in the past 12 months I have been forced to update to two different versions of Elements (2019 due to the mapping issue) and 2020 due to compatability with MACos10.15. 2020 seems to be the worst version of elements - I cannot get it to recognise my 2 cameras and this problem with the latest update.
But again, thank you for your help - it is appreciated.