Easiest to adjust the post down, if possible.
The correct adjustment would be to give the crossbar a very slight bow upwards. Just enough to actually see it when sighting across from side to side. Just not flat/straight, to be sure there IS tension.
As mentioned above, they do get corrosion inside the crossbar that may prohibit adjustment. There is a compression sleeve around the striker post and it can fuse to the striker with corrosion.
Other than setting the post lower which could also be too low and cause the mast base to scratch the crossbar when stepping, you can get at the striker rod adjustments by taking the boat apart. Removing the crossbar end caps and bolt compression sleeves. PITA, but that is how you access them.
Director of Parts and Accessory Sales
Hobie Cat USA