I recently bought a Wilderness Systems Sealution (1995) and I am having difficulty with the seat. No matter what I seem to do my seat slides towards the stern while paddling. At first this is not a problem, but soon the backrest is being pushed under the rear deck and causing major discomfort. I have tried a shim but the seat still slides. I can find no mechanism in the seat that can lock it into place. If you have any advice please let me know. I appreciate your assistance.