Disappearing Tags

I used Mp3tag to organize and tag my entire library of songs, about 1500. But for some reason, there are about 100 songs whose genre always switches to "other" after I have changed them to some other value. The switch happens after I close and reopen Mp3tag. any suggestions? thanks

Are they a different format to the other files? Are they being accessed by some other program? Are updates to other fields accepted by these files?

i seem to have fixed the problem. they were, i think, being used in some program i was running in the background.