Is it a self built rack? When self biult racks have too much span in width without adequate vertical bracing, especially when done with heavy materials like melamine, they often sag in the middle, and tubs near the bottom, especially in the middle, will get "pinched" and stick.

Lubricants are often petroleum based and could cause a fire hazard, damage your heat source, or both.

You may need to modify your rack. If it is a manufactured rack, you should probably call the manufacturer.