It's difficult without seeing your code. But from other people's problems, my guess is that your code has problems with the following test:

isMerge('aaba', 'aa', 'ab') which should equal true since:

s:     a a b a
part1: a     a
part2:   a b

Maybe that will nudge you in the right direction.

If not, I'd be happy to take a peek at your code and give some hints.

Good luck! :-)