First, with the slide in, look at the paint work at the front and the back of the FWS. Does it line up horizontally?
It sounds like your slide is a little "cocked" in the opening. With the slide out, if you look underneath near the outer edge, you'll see some vertical 1/2" diameter threaded rods. They have 3/4" nuts that lock against each other. Those vertical rods adjust the height of the slide up or down. If you find the paint lines up at one end of the slide and not the other, you may need to make some adjustments to correct the slide and realigning the paint. This may help your issue.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor