An air horn sometimes works
It's worth trying, anyway. However, as mentioned previously, the "little stainless thing" that all the boat builders use for vent thru-hulls should all be replaced with open bulkhead thru-hulls. Those stainless thru-hulls are a "universal" thru-hull fitted so they can be used to vent gas tanks--with just a slit to prevent water from contaminated fuel and a screen that acts as a spark arrester. They cause more problems than they solve when used for holding tank vents, because the slit gets clogged very easily, the screen rusts and becomes a trap for dirt, dust, pollen and anything else that gets near 'em. They also contribute to odor because the slit is so small. I hate 'em...but boat builders use 'em on everything--gas, diesel, water and holding tanks.