I threw it back together to check how much I had to hold it down and all I do is hold it in place. It fits nearly perfect. Are you sure your snorkel isn't hitting it? Mine is removed completely, and otherwise is the same as yours. I will also ask if the covering isn't warping the seat pan. Other than that, this is a real real head scratcher for me.
You can outdistance that which is running after you,
but not what is running inside of you