Read the rudder cam FAQ. Most likely the upper casting plate is too far forward.
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. You can also use a small flat blade screw driver to work between the cam and plunger... force the plunger down to unlock the cam. I find that this works very well even if the cams are dry and un-lubricated.
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.
A cam plate too far forward can keep the cam from fully locking. A cam plate too far aft will allow the rudder to slip aft in the lower casting or allow the upper casting to be raised without pulling the cam open. To adjust the upper plate, lock the rudder down and hold the rudder firmly against the lower casting (forward most position). Loosen the upper plate then back it away from the cam a little. You want to seat the plate tight against the cam (in the fully locked position) while holding the rudder forward in the lower casting. Ease the plate forward while wiggling the upper arm up and down just a bit. The idea is to find the point of deepest insert that the upper plate can get into the cam. That may mean that the upper casting is not touching the lower casting. Don't force the plate too far forward as this will begin to force the cam open. When tightening the plate, be sure the plate doesn't move. I usually tighten with the wrench handle rotating forward towards the cam to move the plate forward if anything.