I had a similar problem. Some , all or none of this may apply to your issue. My centerboard
trunk has 3 rubber shims that run in tracks on each side of the trunk. Rubber swells in water
over time and this resulted in mine being jammed.
Unless you have a very high aspect ratio center board the top of the trunk is probably above the water
line and should have some sort of removable inspection
plate. I was able to get a bottle jack btw the top of the trunk and the center board with some wood shims and jack it down. 4 shims had to be removed from above at the inspection
plate and two had to be removed from underwater. I cut them down and replaced them but have since bought some stock UHMW and have cut new ones that should not swell.
Other options for getting the board down would be to get a line over the board from underwater, secure to a fast point on the bottom at low tide and wait for the tide to come up or do the same thing in the sling during a haulout. You heed to be sure the rear sling is aft of your trunk when you do your pick.