We had 5 years of this! Eventually I made a complaint to the head of service for the paed doing the assessment, as she clearly didn't take our concerns seriously, despite her colleagues agreeing with us, and made up her mind after a 10 minute appointment! She accused me of being an over anxious mother and that my daughter was not classically autistic. I never said she was, I believed she is on the spectrum and was at the Asperger's end, and not the other end.
Took 9 months to get an appointment with the best in our area, following my complaint, and another 6 months of working with her, but we finally have a diagnosis, and it is Aspergers as suspected.
My advice would be to stick to your guns. You are their mum and know them best. We also had issues as no real problems at school, and therefore difficult for them to intervene and support us (because the school routine suits her).
Can you contact PALS or similar for advice of who best to complain to/ask a second opinion from? It might all depend what is available in your area? Can you ask for some advice from the national autistic society?
Ultimately, don't give up. It's a long, hard road, but if you feel like me, you need to know for sure that you've done everything in your power to get to the bottom of the issues, whether that is a diagnosis or not, but to know that you have helped them the best you can. Good luck!