After threading all of your pairs, pull up the slack via the last pair first. It will save trouble when trying to insert the cover. You also need to take care not to bend the wire forward (toward the screw) to much or you can cause crimping and possible damage.

Did your crimper come with a centering head?. If so make sure you calibrate the head after about 20 or so connectors. It will minimize damaging heads while crimping. As you know the pins in the head itself are very thin and prone to "crushing"