I bought recently Jackson guitar and decided to change strings. I cannot screw back in place a clamping block for 6th and 5th string on my locking nut. When I unscrewed the clampig block it took some effort to move the screw, but now, I cannot get grip with clamping block screw . I checked and clamping block is set correctly and string gauge is same as on strings on guitar when I purchayed it.
It sounds like you might have stripped one of the threads. On a recently acquired second hand instrument it is also not impossible that the seller knew about this and just glued the screw in place or it could just be the effect of corrosion on a screw which hadn't been moved for a long time.
The first think to do is have a look at the screw and see if there is any obvious damage to the thread, you could also try one of the other screws. If it is the screw which is damaged its just a question of getting a replacement. You just need to work out the correct thread size (I don't know if it would be metric or not but it should be easy enough to find out from the model number).
If the thread in the nut block is damaged it may be possible to restore it by very carefully running a tap of the correct size into the hole.
If that doesn't work it may need a replacement nut block. A good guitar repairer should be able to do this for you for not too much money. If the nut block is screwed on you may be able to do it yourself.