Join Date: Aug 2005
Location: Vancouver, Washington and Ocean Shores, Washington, USA
Bikes: 2 - 2007 Custom Bike Fridays, 2 - 2009 Bike Friday Pocket 8's, Gravity 29'er SS, 2 - 8-spd Windsor City Bikes, 1973 Raleigh 20 & a 1964 Schwinn Tiger
Suggest you read the instruction book. The hex-shaped rod - about 3" long - that is exposed when the latch is open, can be adjusted (tighter or looser) with a very small hex wrench (used to call them ignition wrenches; used to install/adjust points in automobile distributors).
Doesn't take a whole lot of movement to adjust it - don't over tighten, either. Should result in snug, secure fit once stem is locked in "up" position. It's NEVER going to be totally "non-flexing".