I had hesitated to offer a suggestion, because in this instance all I can do is to make an intelligent guess. But I see that no-one else has chimed in, after nearly three weeks; so my guess may perhaps be helpful.
It seems to me that the problem may perhaps be an accumulation of hardened mud etc inside the case.
If that is indeed so, the only solution will surely be to clean it out. It may help to remove the board first; then use whatever implements and techniques you can to clean out the mud. Perhaps pressure wash, augmented by a suitable scraper (you may need to improvise, or make your own), or stiff brush.
Without seeing the boat that is the best I can come up with.
Oliver