Looking for a bit of advice. This morning our upstairs toilet started draining very slow. I tried plunger it to no avail. The toilet has a AAV stack to the rear and also an inspection cap. if I open this cap the toilet flows ok, and there is no blockage visible. There is also a inspection cap at the point where the pipe goes underground, again taking off this cap the toilet flows fine. however once the caps are back on the toilet backs up again. The toilet gargles and so does the upstairs sink when the toilet is trying to drain. Any help to solve this problem would be great