I have an old Hobie 14. I've had a similar problem with the rudder appearing to be locked down but popping up too readily. I installed a new cam but the problem persisted. I discovered that the pin on the tiller is too wide for the slot in the cam, so I ground the cam down to make the pin a neat fit ... but still had the problem. Finally I discovered that the holes in the rudder were the correct distance apart (90.05 mm), according to the spec, but seemed to be 2mm too close together. I redrilled the rear hole and now it locks perfectly.