Have you given your tipa good thourough cleaning? It may need it. Just take the thing apart.
I found out that the Aztec nozzles can come apart into 4 parts: the tan body, the white-ish inner section, the needle itself, and the spring.
To take it apart, pull the white inner piece out of the tan body. Next, pull the end of the needle out. This is NOT the pointy end, it's the plastic end. The end of the needle that I'm talking about is a tan colored plastic that pushes towards the tip. It should come out fairly easily, but don't worry you shouldn't be able to break it. I haven't yet. When you get that out, you'll notice the spring. This simply slides off the needle.
The first time I cleaned mine really good, there was a lot of paint built up in the inside of the white piece and also in the spring and on the needle. I use 91% alcohol and Q-tips to clean it. If the build up is really bad, you can soak it for a while in the alcohol.
I had a similar problem before I did this the first time, now It has gotten better. I clean this way now after every long session, when I'm ready to stop using the brush for more than a few hours. Shooting some solvent through works well for quick color changes, but this routine I use is for the deep cleaning.
Hope this helps.