Progressive shifts may occur during a print run if the pulleys are not sufficiently stressed with the motor shaft. To properly tighten the pulleys, you simply need to stress the pulley studs on the motor pins. Be careful not to over-tighten as you risk damaging the thread of the stud which is relatively fragile.
The motors concerned by this phenomenon of shift are generally the motors of the Y and X axis, nevertheless it is rarer to observe this phenomenon on the X axis.
