Two items to look at
There are two items that could be doing this. The slider hatch will expand in heat and you will need to take that off and grind off the edges that being the case. Make sure to sand the edges smooth.The other which I would loook at first is to see if the slider is resting or being pressed down on the ridges it is sliding on. That being the case, then there are two things to consider. If you raise the fiberglass portion, you should see a rubber snubber on each side that sits inside the drain. Add some washers to raise that sucker off the ridge. The other problem could be the cut on the fiberglass companion way top which may not be cut enough thus forcing the slider lexan hatch down.Check these out and advise what you found. It is one of these problems or both in nearly all cases.Crayz Dave Condon alias Buzzlebutt