That is one of those damned "scalloped" pressure plates, which is supposed to allow an oversized disk in a "too small" cover. The 440-6 70 RR I had used one. 10 1/2" pattern cover, scalloped PP and 10.9" (depending on what you read) disk. Mine, I believe, was slightly misaligned bell. In any case I had a lot of problems with the thing releasing "clean." With use, the disk generates "fuzz" on the outer rim of the disk from normal wear. This evidently catches on the scallops and prevents a "clean" release.
Not sure if this is your trouble, but keep it in mind. You might consider tossing the next smaller diameter disk in there.
What size is that disk?