My Murray hydrostatic automatic mower will not get in gear?

My Murray hydrostatic automatic mower works fine except sometime when i crank it up and step on the pedal it won't move. In the past, I would keep rocking the mower or jigling the up/down latch and eventually the drive belt would kick in. The mower (42 in deck) has been sitting up now for about 4 months. Now it refuses to engage the drive belt. Can you advise???? Make = Murray, Model 287707, type = 0225 01,
Briggs & Stratton Engine.