Help! Dampening bar not dampening evenly.

I can't really understand how you can be happy with your purchase iif the dampening mechanism on your instrument is not functioning properly.
My advice to you is to contact the dealer from whom you bought the instrument and complain! It's their responsiblility to provide you with a decent instrument.
If you can locate the problem and fix it, fine. But you can't. I might add that this problem is not uncommon with Musser instruments.

It sounds like the rails may be warped. If the rails curve down in the center (i.e., they are concave), the middle will dampen before the ends. The bar pins would still all be the same height off the top of the rail, so measuring them would not reveal this as the problem. The way to check for this is to hold a straight edge from one end to the other. It sounds like the rail is just a bit concave. So check for this kind of warping with a straight edge, and you might find about a 1/8" dip in the center. The effect would be happening on the inside rails, as this is where all the work is going on between the bars, the damper bar/pad, the spring (which must support 1/2 the 30 lbs of bars) and the pedal.

If that is the problem, solutions could vary.
-You could take the inner rails off and shave a touch off the contact between the rail and the side piece. Or you could heat and try to re-bend the the rails so they are once again straight. After doing this, you would probably want to add diagonals running from the center of the pedal to the cross brace for the legs. The reason these warp is because when you operate the pedal, you are creating intense pressure on the rails. This mechanical force should be redirected to the legs. Take a look at how the Yamaha is braced to see what I mean. The Musser design is mechanically inept. You could also add braces running from the center of the inner rails to the lower edge of the legs, which would transfer that force to the legs.

-You could raise the center bar pins so that the ends touch at the same time as the center bar pins

-You could order a silicone-filled damper pad from vanderplasbaileo. This should resolve the problem unless the warping is really severe and is the first solution. (The felt pad used on most commercial vibes is garbage.)Really, you should get one of these regardless. It may solve the problem without the mechanical fix described above (though it's still a good idea to fix this problem). It has most of the benefits of the Piper Vibe bladder-based pad.

-You could make your own liquid filled damper pad using a bicycle innertube. This provides a damper pad like the one used on the Musser Piper vibe. It enables excellent sound on notes played over the pad and, like a water bed, evens distribution of force on the bars while absorbing shock.