This may be related to the upper casting / cam plate position (assuming the boat is mid 80's or newer... if older, it will be a rudder drilling issue). If the cam plate is too far forward, it prevents the cam from fully locking, you get what appears to be a locking cam, but it trips too easily.
Hold the tiller arm up, lock the cam down with your thumb. Look at the cam from the front of the casting. There is a small rectangular opening in the front side (under where the tiller arm would normally be). Mark the casting with a scribe to align with someting on the cam. There is a sharp edge on the cam to use as a guide for a properly rotated and locked cam.
Open the cam with a blade screw drive. Lock the rudder down in the normal fashion, with the tiller arm.
Look again at the cam through the small opening of the front of the casting, under the tiller arm.
Likely the cam is not fully rotated and locked.
If this is the case and you have adjust the cam plate aft. Loosen the screw / bolt holding the plate. Move the plate all the way aft. See that the cam rotates and locks fully. Hold the rudder forward in the lower casting. With the tiller arm in the normal locked position, move the plate forward slowly until it contacts the inside of the cam hook. Lift and lower the tiller arm slightly to be sure the plate can seat fully in the cam (may be that the tiller arm will be slightly lifted off of the lower casting). Tighten the cam plate. Check the cam lock and rudder slop. If the plate is too far aft, the rudder can move aft under load. It will be a balance between holding the rudder forward and allowing the cam to lock fully.
I have seen systems where the plate is not properly aligned with the cam hook. The cam rotates over and hit the top of the plate and cannot lock. I have also seen systems where the castings have to be filed down a little to allow the upper casting to propery nest with the lower and get low enough to engage the cam hook.
Anyway... lots to look at.
