Those bolts should be tight, but not so much that it crushes the ears of the shackles, creating a bind on the spring and bushing.
That nut seems to look like what a typical nylon-locking nut would be. If that is in fact what it is, tighten the bolt up so that the threads protrude through the nylon-locking portion of the nut, but again, do not tighten it so much as to bend the ears of the shackles into the spring/bushings, as this will prevent the shackle and bushings from flexing, as they should.
If after trying to tighten the bolt and nut the threads do not protrude through the nut you will need a longer bolt.
If after trying to tighten the bolt and nut, the nut can be threaded onto the bolt without the use of tools, ie by hand, then the nut must be replaced.
