Can't charge AC

I have a 2002 Tacoma 4x4 SR5, the AC is cool, but not cold. So, I figured I would try adding some R134A.

When I hook everything up, the gauge is reading 15 psi with the AC going, the AC clutch is engaged and stays engaged, but the pressure doesn't rise above 15 psi. When I rev the engine with the AC engaged, the pressure will drop down to 5 psi, and then raise back up to 15 psi at idle. When I turn the AC off, the pressure rises to 50+ psi.

But it won't charge at all, no refrigerant enters the system. It seems like the connections are all good, gas comes out of the can when I test on another vehicle, it works fine.

The compressor seems to work, you can hear the load on the engine when I engage the AC.

Do I have a blockage somewhere? Any ideas of what to try next?