Be sure to scroll down... there may be more than one question on this page!
Lazy susan's can be a pain to adjust! The major problems are that the center moveable shaft may be binding, or the circular shelves have slipped down the center shaft, and are dragging on the bottom.
You will have to be my eyes and look at the center shaft. There are a number of screws and/or bolts. There is one bolt that controls the length of the center shaft by locking the position of a metal sleeve that slides inside of the center shaft. This bolt is usually located near the top of the shaft. If the cabinet top has sagged, it is possible that the shaft is binding due to the pressure. You will have to loosen this screw/bolt and slide the adjusting rod into the center shaft slightly... just enough to allow clearance but not so much as to have the lazy susan fall over.
This adjustment may also be at the bottom on yours.
The position of the shelves should also be checked. Each shelf moves independently of the other... unless you have the type of lazy susan that has the front decorative panel attached to both... then they must be adjusted together. Lets assume that you have this type of lazy susan. If not, you only have to perform this adjustment on the bottom shelf.
Look for the locking screws, located on the shelves in the center. Loosen both screws and raise the shelves together so that the bottom shelf clears the base of the cabinet. This is usually a difficult move, because the two shelves tend to want to bind on the shaft. Don't under any circumstances lubricate the shaft, or the screws may not hold position! Tighten both screws.
As an aside, many folks have a similar problem to yours because they also store very heavy goods on their lazy susans. If you could lighten up the load a little, or at least keep lighter items on the bottom shelf, which usually is the troublemaker, you may forego a repeat of this repair.