1st of all, if it is a backing plate or plate/nuts, you may be able to loosen it up and slide it in and out a little, or the nuts/backplates may just spin, even trying to back them out. If either is the case, you should tighten it back up the best you can using the tips from the previous post and call it good.
If it were the fore or aft hooks you were having issues with, you could access the backing plate by removing the seat pads and inspection ports beneath them.
It is probable that it is brass inserts on the middle hooks, and that they could be stripped. Remove a screw and pivot the bracket around to have a look. You should be able to tell if there is an insert in there. If it is stripped, all is not lost. Look into a heli-coil thread restorer kit, see here
http://www.helicoil.in/helicoil.htm
Let us know how you make out.