I had the same problem with my biocube skimmer. I was either getting no bubbles at all (valve totally closed off) or it was bubbling over and filling the cup. There was no in-between, even when I positioned it higher in the water level. I actually had to replace the valve (cheap, was like 99 cents for a 2 pack) and thought I was going to make an extra cut in the line and use 2 valves to get better control, but just popping on a new valve worked.
It's decent for the price. I get scum and scummy water pretty regularly. After I do a really good cleaning and decent water change, the bubbles actually drop off for a day or so, but the eventually pick up again.
Once I changed the valve, I had to move it up and down in the water level a bit to get it right, but then it worked fine.
I'm actually upgrading the nano remora, but not because the biocube one wasn't working... It was a decent starter skimmer for a great price, but I'm ready for a really good one.