How do I file for VA disability if they keep denying benefits?

I've filed for disability several times. I was hurt while I was in active duty. I have my original Dr notes from the Navy but they keep denying benefits to me. The VA put me on pain pills for over 6 years and nobody will help me. The attorneys say I need a "nexus" letter but I'm clueless how to obtain 1. Is there anyway to speed this process up. It's been 16 years with no benefits from being disabled.