I have used Monroe air shocks often and never had a problem. I definitely think your problem is the angle of the shocks. There's no where for the shocks to expand.
Straight up and down of course, you get the most and equal amount of travel out of your shocks, but angled toward each other even a bit, you then only have travel until they both start trying to bend the mount surface inward. Of course, your mounting crossmember is too strong to bend so,..the shock stops moving upward.