Is the spring seated correctly in the lower perch/bucket? The tail of the spring should sit nicely into a channel, allowing the retainer to be tightened down.
If you look at your lower bump pad relative to your bump tower, it's fairly obvious what's going on. If you don't have a spun or bent axle tube (which should be pretty easy to rule out), your axle isn't centered.
Do you have side pictures of your rig? What about wheel base measurements?