most people have a designated section for their skimmer. this allows the water level to be consistent no matter what level the return section is. it keeps skimmers supper consistent, and allows people to "Dial a skimmer in" in your situation with fluctuating water level, you will never reach the full potential of a skimmer.......
you might think of silicone-ing a baffle in your sump (preferable where the dirty water comes in the sump) so that you get a more consistent water level...
just a thought.
also, i had a skimmer do what your talking about, and the only fix for that is to raise your water level. with a skimmer pulling super dry foam, it becomes super sticky, and the proteins will not collapse. no matter how long the inner ring of the lid is, or even if there is silicone grease.. (eventually the grease will get covered in foam and your back to square one....)