I have had some problems with my CPR in the past. Here are a few things that I found that solved my problems.
Problem: No bubbles forming in the chamber OR only macro bubbles forming in the chamber.
Solution: I adjusted the inlet tube. It is the black "L" shaped piece that attaches to your pump. Making small adjustments left and right produced more of less bubbles,depending upon which way I turned it. I dont know why this works, it just does. Be careful not to tighten the inlet "L" shaped piece too much, IT WILL BREAK.
Problem: Not producing any bubbles OR macro bubbles.
Solution: Ok, you know the airline tubing that connects to the inlet of your pump? Well on some of the pumps that come with the CPR, the tube is connected to the top of the cylinder where the water enters the PUMP. Now, When you look at your pump, look where the water enters the pump. You will see that it has a "Mercedes" type symbol or something of that nature. Simply take your airline and stick it in one of the slots. Your tubing should now be stuck in the front of your pump, NOT THE TOP like it previously was. This fixed my problems and it produces billions of micro bubbles.
Problem:Micro bubbles, but no junk in the collection cup.
Solution: Either your tank is really clean, the skimmer is "breaking in", or the seal around your collection cup isn't tight enough. Your collection cups should of come with a black elastic band thing. Make sure that this is forming a tight seal within the chamber. If it is not, the skimmer may not collect foam.
My CPR pulls about half a cup of junk every four or five days from my sump, they arent bad systems when you can get them to work. Let me know if you have any problems