I occasionally have the same problem with mine. From what I've read and heard these skimmers just need to be constantly monitored. I check mine once a day to make sure that I have the right flow.
When you start the skimmer start it with a real low flow, check it 20-30 minutes later and adjust the flow higher. You need to keep this up until you just barely see bubles at the top of the collection pipe. It doesn't look like they are entering the collection cup but they are.
If your flow is to high you will get either bubbles or an overflowing collection cup.
I have heard others recommend putting a filter sponge in the return path but I haven't tried that. I would think if you do that then you have to be careful that it doesn't become a nitrate trap.
Hope this helps.