Please watch the setup video for the AT-LP120-USB Turntable following this answer, paying particularly close attention to the section on balancing (or zeroing) the tonearm prior to setting the actual tracking force.
I suspect that there is too much downforce on the stylus assembly which is pulling the magnet for the left channel out of alignment with the pole piece that extends from the left channel coil located inside the cartridge. This is a magnetic type cartridge, so proper alignment of the dual magnets on the cantilever to the pole pieces extending from the coils is crucial for proper operation. Balance (aka level or zero) the tonearm by rotating the entire counterweight assembly in a clockwise or counterclockwise direction. This causes the weight to move backward or forward on the arm. The goal is to get the arm to float perfectly level in mid-air. this typically means the arm will be about 1/8" above the tonearm rest and parallel to the record. To "zero" the arm, next turn the black stylus force gauge ring ONLY, not the entire weight, to zero.The ring turns independently of the weight. Now to set the actual tracking force, rotate the entire counterweight assembly counterclockwise so the numbers on the ring go up in value, and stop when the ring shows the recommend tracking force. For the provided AT95E Cartridge (green colored stylus) this is 2.0 grams. Set the anti-skate control on the side of the tonearm pivot to 2 as well.
AT-LP120-USB Turntable Set-up Videohttp://www.youtube.com/watch?v=YCv7