Checkboxes not working in browser view

Hi everyone,

I've got a PDF form which, when it opens as default in a web browser, is behaving very strangely....

The text boxes work fine to input text, but there are a number of checkboxes which, when you click on them, have a black square initially to show it's been clicked but as soon as you click away, it disappears and there's no 'tick'. However a couple of the boxes, sometimes (randomly it seems) WILL tick correctly - but then you can't un-tick them again.

I've tried in Edge and Chrome - same result in both.

On the desktop - opening natively in Reader - it works fine.

The initial thinking was that it was just some sort of browser incompatibility. However, when we try the same thing with another form with tick boxes, that one works perfectly fine. Which is leading me to think there must be some sort of difference between the way the two PDF documents are formatted.

Unfortunately I don't have a lot of experience creating forms in Acrobat, and I didn't create the form myself initially. However, as I have access with CC, I'm keen to see if there's something obvious that might be causing this? Is there something I could checking in Acrobat to make sure the checkboxes are set up correctly to work in in the browsers?

Thanks so much in advance for any help.

Danielle