Can you move it rearward and then get into the trouble causing spot with some kind of tool? That stuff is all pretty soft aluminum, on my stock rack at least. If the metal gets dented it seems like it really easily cause a "bump" that the clampie thing can't get past.

If you can't move it rearward either, I'd think it's metal to metal seized.

You can replace the bolts and sliders and knobs with pretty standardized 1/4-20 bolts and those "daisy" (or other) shaped knob/nuts from woodworking places like Rockler. Regular 1/4-20 bolt heads (7/16) fit in there, the oval, oblong, flat heads of the ones you get at Rockler fit even better. Not expensive.

I'd guess that those crossmembers aren't really meant to be adjusted a lot over their lifetimes. Surely most people adjust them just once, if at all.

Lump

 

Thanks for your input, Lumpy.

The crossbar moves forward and backward along the rails until one end hits a hidden and inaccessible obstacle at the mount. From what I can see, the rails and crossbar (which hasn't been moved before) appear to be in perfect condition.

 

I was able to move my van to a spot with much better light today and found what the problem was.

The rearmost screw that fastens the rail to the mount wasn't quite screwed all the way down so its head was blocking forward movement of the crossbar end. Now that the screw has been tightened down, the crossbar end moves freely in both directions along the rail.