and
Make sure the output is not under water,it cut's easy enough for your application.I could see the back pressure doing what you say.I have three a nd have not had that happen,also the bubble defuser [if you use it]could clog also causing back pressure.O ring thing could be possible.One way to know for sure is to see if water is comming out of the small vent hole on the out put.Its located at the top of the tube.This would tell you something is blocking the flow.