The easiest way to clear this us is for you to post of photo of the part that you can remove. If you can add washers on either side it will increase clamping force (it effectively makes the clamped space wider). If you cannot add washers, you need to reduce the length of the shoulder part of what you can remove. If it is the nut (the female part), getting a shorter nut would work (and if you cannot find a shorter one you may be able to shorten the nut with a grinder). If the only thing you can get out is the bolt (the male side), you either need to add washes or find a bolt with the same thread and a shorter unthreaded shoulder.

I also note that on many bolts for this location one side has a key to prevent it from turning (a little protrusion that fits into a notch in the clamp), perhaps that key has become jammed into the clamp (I have had this happen when they were put in the wrong side). All of these potential complications is why I would try to see if you can get things working with either shims or washers, then at least you know that the post and clamp are going to work, while you try to determine what parts you need to replace.

-Will