If the belt is loose, it can definitely cause that.
The easiest way to adjust it is to loosen the two set screws on the bottom side of the Tail Block. The 5/64″ (ball-end) wrench included with the AxiDraw is the correct one to use, and you should need to use the short (straight, not ball) end of it to loosen or tighten those screws. This will loosen the Tail block from the 6 mm (Y) shafts; you can vary the tension by shifting the exact position of the Tail Block with respect to those two shafts. When you are done, tighten both screws securely with the short end of the wrench.