Works on java emulator, not in reality!

what might be the possible reasons for a j2me application to work on the default sun wtk emulator and not on any real cell phones?
also what are the possible ways to solve this.

i used a nokia 3110c & motorokr e6 for real tests. they both show invalid application errors.
its a simple hello-world application working perfectly on the emulator.

i am sure both the phones qualify for the necessary requirements required by that app.