I'm playing with patch 1.06d, and I've had trouble commiting units a couple of times. The last time it happened, I attacked the Russian 1st army with the German 8th army. I had 3 infantry corps in my stack, plus another 3 infantry and an artillery corps in reserves. After commiting an infantry unit and fighting the first round, I brought out my artillery from the reserves, but I was unable to commit one of my remaining 2 infantry corps to combat. When trying to drag either of them into place, I got an error message saying that the units had probably suffered a shaken result on the previous round (neither of them had been involved in any combat so far).
After trying to drag them into place a couple of times, the battle ended with my defeat. I guess the game decided that I had no more units to commit.
Am I doing something wrong here, or is this a bug? I don't think I encountered this with any of the previous patches (although I haven't played the game since 1.05-something).