After messing with mine for a year, I found what seems to have worked. It is the bottom track (which has the seal strip in it) being not parallel and not the right distance from the door bottom. I removed the bottom frame from the wood and built up the low end with small strips of wood. Upon reinstallation, I sealed the INSIDE portion to the fiberglass. I also put a dab of Clear silicone at each end of the seal itself, sealing its ends to the fiberglass wall.
This has stopped 99% of the leakage. If I still "drown" the area, a few drops will get out. It's just not a very good design seal.
I also keep a plush 100% cotton bath rug right up against the sill so any water that gets out is wicked up right away.
"If the women don't find you handsome, they should at least find you handy." - Red Green