If you can't get PBblaster in there (it's pretty tight) you may have to try the following steps.
1)Try to unscrew it
2) Try to tighten it
Basically you're wiggling it back and forth. The idea is that if there is some corrosion and or debris that has managed to work it's way in there (contrary to popular belief stainless steel does rust) this will help loosen it up. If there is dirt and debris in the threads, simply trying to straight up unscrew it can cause the dirt to ball up and bind the screw completely. If it starts to unscrew remember to tighten it back every so often BEFORE it starts to bind. If you manage to get it to back off a bit spray the PB Blaster in there and let it soak. I would put a rag down under the bars so that you don't get PB on your paint.
Some of the other guys might have some thoughts.