You may be making the same mistake I did when I first installed my Tubus Cosmo. There are two barrels which the horizontal stays go through on the underside of the rack, right? Well, what I did was put these in backward, so that the nut on the side of the barrel could not tighten the stay. It's hard to describe, but basically look at those adjustment barrels (under the top of the rack, which the two stays go through from the seat stays). These can be mounted the right way, where tightening the nut on the side of the barrel will secure the stay, or backward, in which case no matter how you tighten the nut the stay isn't being clamped on. I only realized I had it the wrong way around when my rack suddenly swiveled backward on one of my test rides. I was embarrassed to say the least once I realized my mistake, but it's good to know someone else was taken in by this too!
The allen bolt on the top of those barrels only keeps the barrel in place side-to-side in the slot, I don't think it has anything to do with keeping the stays clamped.
Hope that helps,