Cracked ipa's not working??

I've downloaded several different games, even different versions of those games, and sometimes the cracked ipa just won't work. Most do, but the odd ones here and there won't work. Is there anything I can do to make them work?

I try them on 3 different iPod touch's, each with a different OS (3.1.3, 4.0 and 4.1) and none will work. Any help would be greatly appreciated!