Instead of the silicone spray (attracts sand, not environmentally friendly, and doesn't seem to be working
for you) try a bar of soap on the rear of the centerboard and rubbed along the edges of the gasket.
If the problem persists, VERY carefully trim a slight radius on the rear section of the gasket lips to widen the slot between the lips.
Also, if you can, carefully fold the gasket up nearly vertical and try to put a 45 deg. bevel on the
upper inside of the lips.
The reason it sticks is that the rubber/plastic is pretty hard (makes it durable, but not very functional)
and the rear of the CB is at the wrong end of the leverage equation. 6-8" from the pivot for the handle
32" + at the rear of the CB.
All of these are suggestions I have used on Starboard CB's that stick!