When a new version of MAME is released the romsets are rebuilt to work with it. Is everything affected, no, but given enough version releases the number of titles affected also increases. If this weren't the case then there would be no reason whatsoever to ever update the sets with the one possible exception of adding titles. That said if you are one or two releases off, probably not going to be a big deal.
If you are using clrmamepro to create a dat and rebuild your sets to an older exe, then you shouldn't have any trouble as long as you have the rollback to do it with. Still, you are giving up a lot of advancements using such an old build. To each his own, however. Admittedly I haven't bothered with this method in years now.
Still, there are some settings in the integrated mame section of gameex that have direct corelation to missing titles.
Test some things:
Set "verify roms" to false
set to allow imperfect....etc... and update GameEx upon launch to see if those you're looking for have returned.
Advance MAME settings