This is usually caused by the latch mechanism in the drawer. It will probably either be very worn, or screws etc coming loose. Try taking the drawer apart and tighten/ lube the latch that holds the back of the drawer tray.
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If anyone is looking for a Casio 140CR programming and operation manual, c-tech collective have it here:
http://the-checkout-tech.com/manuals/cashregisters/Casio_140CR_user_manual.html
Registerman, September 2010