I string with an Alpha Pioneer DC plus, however I've been noticing that when I'm strining that sometimes the raquet gets loose, because the knobs on the head and throat of the racquet need to be screwed tighter later on. I believe it's happening when I"m doing crosses, and I always double check to make sure I'm mounting the racquet correctly. I just notice over time of stringing a racquet it slowly becomes loose, and then I have to retighten the knobs at the top and bottom of the racquet. Also sometimes near the end of the crosses when I'm pulling tension I notice that after a pull, the racquet all of a sudden shifts to the right or left depending where the string is being pulled, which is when I usually notice that the knobs are loose again. What is causing this? I'm pretty sure I'm tightening everything fairly well.