I've recently sold my WB (sail no:95) and I also had problems with sticking plates. You will get more information if you search under 'bilge boards' which is another definition for this plate!
In my case the boards stuck through rust forming on the iron weights causing the metal to blister and swell. I was able to remove the rust and thin the metal by using an angle grinder (with care). From memory I could have removed the boards by loosening the fixing bolts at the front of each board with an Allen key. Some of the early WB's did not have this facility for taking out the boards. The boards can also stick if warped or if the rope use for raising and lowering has been replaced and the wrong knot used and is wider than the boards thickness. I think the housing for the boards narrows at the top so if the boards are pulled up with force they get wedged. Hope this helps. Best, Mike.