Yes known issue in 421 I have seen that before.
There is not enough detail obviously in this request to pinpoint to the exact DDTS/smu that you are running into, but I want to make 2 recommendations here:
either apply the latest smu's for 421 (use Cisco Software Manager/CSM) to help with the SMU mgmt as I realize that is a PHD task on its own, or consider 423 with its latest smu's.
regards
xander