Trouble with grey tank dumping using garden hose

Air usually comes from up above the roof (there should be 2 or more vents up there,, They look like a pipe wearing a hat). Sounds like it is either:

A: Clogged (run an electrician's fish tape or a plumber's snake down it, Be advised that they do not always run STRAIGHT down into the tank, sometimes they have elbows in the line (usually 30 or 45 degree ones) to offset around an obstruction or bad planning)

B: Pipe has fallen down into the tank to the point where it's efectively clogged.