Ah, so I'm not alone. I thought it may be something to do with size of blob in that ones with three people open but not ones with 23. However, some open on four, others not on four.
Don't suppose there's any point trying to figure out the oddities of Ancestry's workings. I'll leave it a week and then try the equally pointless option of contacting them (to be told to clear cookies, try another browser, etc., etc., etc.).
They did once tell me that some operating systems take a few weeks to catch up with Ancestry's changes so maybe that's it.
Thanks for the replies.