Why do my windows drop into my door with every bump?

Why do my windows drop into my door with every bump? It seams as though every time I hit a bump, my rear two windows drop further and further into the door. The window switch to raise the windows make a sound as if they are working but the windows don't seem to catch and raise up. Once parked, I have to use my hand to press against the window and raise it up myself. Besides using cork screws to hold my windows in the closed position, how can I fix this issue?