You should review the FAQ on cam adjustments. It is possible that the upper plate is out of adjustment. Since you say the rudder hitting the beach is not kicking it up, it makes me think that lifting the arm is not unlocking the cam and that is an adjustment issue... or lubrication issue.
The Hobie rudder cam system is pretty easy to deal with if you keep it greased. Use marine grease, if possible, between the cam and plunger. Anything will do as a temp lubrication: Suntan lotion, WD-40 or Chap Stick etc. Marine or bearing grease just stays there longer. Lubricating the system will prevent damage to the cam and plunger that is caused when the cam gets stuck in the down position.
A tight fit is required between the upper arm and cam (lower casting) when the rudder is locked down. The rudder should be held firmly against the lower casting. Any rudder movement, aft from the casting, indicates a miss-adjustment that can allow the upper arm to disengage from the cam without forcing it to rotate into the open (unlocked) position.
If the cam sticks in the down position there are several methods to get it to rotate release. From above and forward of the assembly, lift the upper arm and rotate it aft and out of the way. Loop a line around the cam… yank the line to pop the cam open. You can also use the tiller arm to assist this technique by wrapping the line around the cam, then lowering the tiller arm and wrapping the line a few times around the tiller arm. Pull up on the tiller arm which (through the line) levers the cam open. There are several tools that can also be used. I use a large blade screwdriver that can be inserted into the side of the cam to leverage it open. There is also a tool (Hobie Part # 83103 / 2003 Catalog page forty eight) that has a small hook that can be used (by drilling a small hole in the cam to allow it to be inserted) for leverage.
If the cam is really stuck down, the only tool needed is a small blade screwdriver. You work the blade between the cam and plunger to force the plunger down and release the cam. Don't bother with the big screw under the spring they often are fused in place and the spring does not need to be adjusted but a few times in the life of the boat.
If the cam keeps getting stuck down, even when greased, there is a miss adjustment in the upper casting plate (newer boats), the rudder is drilled wrong causing too much play in the system or it is worn out (too flexible) where it hooks the upper casting.