Keeping the AI rudders down has always been a challenge. The Twist & Stow rudders originally relied on an external notch to lock down. Proper pivot bolt adjustment was critical to locking as well as rudder retraction.
At faster AI speeds, this was insufficient, so a screw-in locking pin was added. It would be sheered off if rudder kick-up was required. This worked somewhat, bit didn't always solve the problem.
Pull cords (with cam cleat) were introduced in late '08 (I think), replacing the old up-down lever. This improved the rudder's up/down operation and the ability to secure the rudder in the down position. I think it's still supposed to be able to kick up under force, but have not tried it.
An internal detent was developed to help lock the rudder yet allow it to kick up if necessary. I think this began with the '09s.
It's been a tough problem, partly because of the oblique retraction of the T&S design; turning to starboard somewhat mimics the up/down action. IMO, there also seems to be a performance variation between boats. Part of this might be still explained by differences in pivot bolt position. I also noticed the rudder assembly can get a slight warp from sitting in the sun after a year or so -- enough to make a difference in how well the rudder stays down.
So if you're having problems, you can try replacing the T & S assembly with the latest version (with detent). To be honest, I can't say that it will solve your problem, but should make an improvement. They all look the same from the outside, so I would recommend requesting that your dealer order one from the factory rather than sell you one from existing dealer stock.
Hobie is always making improvements so I would look for Matt to correct this info if it is not accurate or current.