Typically a skimmer not skimming is usually an indicator that it is dirty. But since you said you cleaned it, there goes that theory. But I'll ask these questions anyway just in case.
Is the injection spray free of clogs and debris? You may need to unscrew the white screw and have look in there. My Aqua C came with a little brush so that I could clean out that area real well.
Also, is the collection cup too high? This may be the problem.
What about the powerhead? That clean as well?
Have you added or changed anything to your tank in the last few weeks? Meds? Supplements? Overfeeding? Etc...