Over time of inactivity, the arm lifter can get sticky, not allowing the tonearm to go all the way down to make sufficient contact with the vinyl. 

The easiest way to solve this is spray a little bit of WD40 on the base of the shaft, and holding the shaft (not the arm as it can break with excessive force) and work the shaft up and down a few times to work the WD40 in. This should free up the shaft to allow for normal operation.