I have had this happen to both of the GS6000s I work with. Remove the end cap on the control panel side. Loosen the screws that hold the motor in place and pull it toward the rear of the printer adding tension before tightening the screws back. The tension is supposed to be around 3.5 kg - I didn't have a spring scale big enough so I just winged it and it worked. Worse case scenario is that you will have to do it more than once.
edit: I thought I'd mention that before the newer of the 2 printers I work with was out of warranty I called a tech for this. The tech never had a spring scale so he winged it also. His fix job lasted about 5-6 months. Epson also had him swap some parts that I'm sure we didn't need (encoder strip, CR reduction pulley, CR belt).
