Try question 9 in the leJOS FAQ
In particular look at the thread that is linked from that question. It explains how you work out which method you are getting the error in. The details in the FAQ are only correct for 0.85 ofleJOS
If you need more help, you need to tell us more about...
1. What version of leJOS.
2. Show us your code.
3. Explain what you expect to happen and what actually happens.