Could be "sand locking" a common problem in shallow waters when enough grit, silt and sand gets into all the reverse bucket points it can jam up a bucket something feirce. Its actually a major reason why alot of folks here either dont run reverse or at least only use it as a last resort. You get stuck in reverse in a fast flowing section of river and you can be in big trouble.
Other than whats been suggested here already if it is just sand locking alot of guys here will drill and tap the bucket with a small single bolt and attach a small length of coated cable which they attach to another part of the transom. This gives you something firm to yank up on the bucket. Its happened to me a few times and it feels like something mechanical is holding it back it can be so stuck, you are nearly 100% convinced the cable is pooched, but its just sand etc