Got salt, eh?

It's not surprising. The bolt faces downward and is at the very rearmost of the torsion bar. If you can't turn it, it is time to start soaking it in some sort of penetrating oil (Kroil, PB Blaster ect) for several days and then try again. From the looks of your body, I can promise you the adjusting bolt is rusted and it is going to take some work to get it freed up.

Good Luck,
Michael