LiE, yes, I showed that to someone on here a couple of weeks ago because a friend of mine has one. Don't remember if the person got it though.
DA, it could be a few things. Most likely, you have week shoulders. Reduce the weight and take things more slowly, making sure you use a full ROM with proper form (back against backrest with no arch).
Alternatively, something could be tight or not working properly and pulling you out of the groove.