Did you check wether the cheat was for you're country because certain codes only work for certain country's.If that doesn't work then you might have to try downloading it again.If that fail's don't bother.

I've also had trouble with the cheat.dat files on newer versions of MAME. Best advice is get an older copy. I did have 0.98 and tried to use the infinite time cheat and it didn't work. I got 0.67 and 0.64 (for Kaillera), and that cheat worked.

Mame changed over the years. Cheats that work in 64 and 67 and others around there don't work in the newer versions.