AC Condensate Line Not Draining
Your condensate line may not have the proper pitch to allow the water to drain.
Another option is a condensate pump. These are little water pumps that pump the condensation water from the drip pan out through the tube. This can be wired to run while the A/C unit runs, or when water is detected.

Out of curiosity, where does your drain line end? At some point I think it is up to high and not allowing the water to fully drain or sagging once it fills with water, then stops the drain from draining.