If the carrier is not pushed all the way down into the power tube, the shims will get caught up in the threads where the power tube tip screws in. To prevent this, push the carrier all the way down (can use a pen or pencil), then with the power tube facing up, put in the shims. Make sure the shims are all the way down on the carrier (past the threads). Keep the power tube pointed up and install the power tube tip.