If you could give a bit more detail about what sort of problem(s) you're encountering, someone might be able to advise. At the moment, you haven't given us a lot to go on!
One suggestion though - have you tried using the
Kindlegen application to convert your EPUB and then checking the resulting MOBI file with the Kindle Previewer application? If that comes out OK, you could then upload the MOBI file directly.