At the moment, I don't know the answer to this. My guess is that it is not the Search button that is failing to locate the relevant pages. We cannot see those pages on the Index page of the relevant board. That, I suspect, is the problem.
Recent posts in the photography thread still appear in the last 50, and the total number of posts on the Info Centre is correct. That suggests that the content of those pages is there somewhere but the page numbers are failing to display. My further guess is that, if we knew the url to the *missing* pages, we would still be able to access them.
I have checked on Proboards Support but can't see anything yet (other than a similar problem that was raised over a year ago). I have to go out shortly but, if the problem hasn't been resolved this evening, I will raise it with them again.
It is a possibility but ProBoards' software should be able to cope with the quantity as I have seen much longer threads on other forums. Unfortunately, there is no such thing as a sub-thread but, if it seems to be an ongoing problem, we may have to consider the possibility of locking a thread once it reaches a certain number of pages and starting a new one (cross-linked).