I would cut the drain above the trap and blow through it, likely the trap is plugged. reinstall pipe using a union so the pipe can be taken apart at a latter date to blow though again.
OR the other problem you may have is that the coil is freezing due to low charge....You will need a PRO to find and repair this problem.
OR the other problem you may have is that the coil is freezing due to low charge....You will need a PRO to find and repair this problem.