Furnace Water Drainage from inducer motor
It sounds like your trap may not have enough height to keep air from being pulled in. I have to make traps that were close to 5" to prevent the pressure from pulling the water out of the trap. Yes they do get dirty and need to be cleaned, but if the trap is holding water it will drain while running, if the trap is not deep enough air is pulled through the trap and water will build up until it is deep enough in the pan or shuts down and gravity pulls it out.