I too had a similar problem. It turned out a critter (mud-Dabber) had built a nest under the lip of the vent outlet, against the screening, blocking the vent. I cleaned it out and darned if he/she didn't come back and rebuild the nest a second time the following week. I cleaned it out again and sprayed the screening with some repellent which seems to have done the trick.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."