I did have the same problem you are having on my second Skimmer I built. I thought that something was wrong with the stone or the pump. Just to see what would happen, I let it run for a few days with the bubbles about 3” from the stopper. Few days later everything was working fine. I hope the same goes for yours. I really think the Skimmer just needs to break in. Let me know what happens.I ran it for about 3 hours and turned it off before I left for the night just in case. I've been tinkering with it and i'll get it to work I was just curious if you had a similar problem. I did try unscrewing the lid on my skim catch.
Thanks for answering. My bioload should be pretty decent so it should get some skim going. Ill play with it more tomorrow.