Condensate drain does not drain properly

Make sure that the vent pipe is the same size as the drain line coming from the coil and is before the trap. Also the trap has to be of sufficient size and has a lot of fall, so any negative pressure remaining after the vent will not cause the drain to reverse and keep the water from draining from the rest of the piping going to the outside.

I had this exact same problem. The installer did not install a trap in the condensate line and also did not cap the cleanout. Once I added one and capped off the cleanout, all has been draining correctly.

Air handler condensation draining problem