Torchy
We had the same problem with Ronin's daggerboard.
Basically the daggerboard case is too long (fore and aft direction) for the board (4-5mm max), this allows the board to rock fore and aft causes it to jam solid. Peter had no hope of getting it up or down once that happened
We have now solved the problem by effectively shortening the daggerboard case. At the top I have made up a hard plastic plate that is screwed down to the deck, this is set so that the trailing edge of the board runs against it, keeping the board right up against the front of the case, we refined it further by adding a nylon wedge at the bottom of the case, this was made from a small length of nylon chopping board, nylon seemed to work well as it compresses just enough to be gently hammered home (therefore not needing any glue). We also had to do a bit of sanding of the daggerboard near the top so it went all the way down
It's proved very effective, to the point in light winds it has a habit of floating up every now and again.
I'll put the rest of the plastic sheet in my spares box, you are welcome to have a bit at Weston or whenever we meet next