Black Tank Vent

Could be blocked...climb up on the roof and check it at the top for obvious blockage. You can also stick a garden hose in it from the top and force some water down into the tank to clear it.

It may even have been pushed or fallen too low into the tank. If the lower end of the pipe is below the level of whatever is in the tank then it won't vent. The pipe will need to be repositioned. higher.