Alot of times it is the type of loop material used. It can also be caused by wax being in between the loop and the center serving or it can be the type of center serving on the bow. If it is tight, (and it should be considering you used a set of d loop pliers,) then it won't move up or down any, it will just be a little annoying for you to deal with while practicing. I wouldn't worry about it too much if you are confident that your loop isn't going to slip.