If you haven't, find out why they are not paying and try and work it out, saying something more than "pay me" can be surprisingly effective, do this through ema

If you haven't, find out why they are not paying and try and work it out, saying something more than "pay me" can be surprisingly effective, do this through email/mail so you have a written record. If that fails, have a lawyer send them a letter. If that fails than small claims court. The first two steps will put you in a better position for small claims court and make it less likely it ends up being your word against theirs, get that documentation.