I like the Xelastec on my S8F. They give me a perfect seal and super fast and tight bass without shoving them all the way down my ear canal but I have to use them in size "ss". Maybe your's are too big in size. Sound's like a bad seal.
For xelastec i use MS and all other M size, even though they don't work for S8F for me they are perfect for smaller shell shorter nozzle iem like t2+ for example.