I purchased a Detonics USA CombatMaster last July which gave me slide cycling problems also. After trying a new barrel and slide, heavier magazine and recoil springs; the problem occurred less frequently, but they still happened. When using longer seven round mags, the malfunction happened just as you described. With Detonics USA mags, the last round would sometimes fail to feed. Their head gunsmith attributes my problem to a rare case of "grip sensitivity".
After everything we tried had failed, Detonics USA replaced my CombatMaster with a new pistol. The new pistol utilizes a commander length slide and a StreetMaster frame and is 100% reliable. Grip sensitivity is not an issue with this gun. They replaced the entire pistol because the CombatMaster's dust cover would have to grow 1/4 inch. Therefore, a new frame is required. When the new model goes into production it will be called the QuarterMaster.
This was done free of charge, since it is a new Detonics USA gun. They also sent an extra magazine to me to cover my shipping cost.

Mikko