Could be a few things. First, make sure the belt is seated properly on both pulleys. Second, check the grease in the main bearing shaft. There should be enough to raise the level just beneath the top of the bearing well after the main shaft and platter have been inserted into place.
You haven't mentioned whether this is an old or new table. If this is an older table or if you have moved the table around recently, make sure there is a ball bearing at the bottom of the bearing well. This small item easily falls out whenever you move the table and are unaware of its presence. An older table might need the bearing grease cleaned out and changed. The belt might need replacing if it has stretched out over time. Or the pulleys may have developed a glaze which prevents the belt from grabbing properly.