A Steep Learning Curve

So I got an airbrush a while ago, and after a few problems with using canned air, obtained a compressor (thanks honey!). Now I'm still having some problems, and I'm not sure what's going on. Perhaps you can help me learn?

I'd heard that the nozzles get blocked because acrylic paint dries to quickly with an airbrush, so I've thinned the paint, and added acrylic retarder to it. Sure enough it does stop the paint from hardening to quick, but I'm still having spray problems. Air is coming out of the brush, so there's no problem there, but it doesn't seem to be picking up anything from the container.

It's not an issue with the paint thickness, because I've got the recommended consistency of milk going okay, and also because it also won't spray water. It doesn't seem to pick up anything from the container. I can't see anything going up the hose. So it's more of an issue of suction.

I've got the nozzle on properly - as if its too lose it blows paint or water back out of the container air hole - and that's not happening. I have had occasional successes with getting it to spray, so I know it can do it, but I'd really like to get it to work every time i use it. At the moment it seems a bit like voodoo when it works.

Any suggestions? It's absolutely magical when it does work, and it's saved me a ton of time doing the red armor on my Blood Angels, and the uniforms on my Imperial Guard. Just need to get it to be reliable.