You may have something caught in the latch that holds the drawer tray. Look inside the drawer to make sure no bills or receipts are stuck inside. Otherwise the drawer latch may be worn or the solenoid in the drawer may be failing sometimes.
----------------------
If anyone needs more information, the sam4s ER-150 manual is here:
http://the-checkout-tech.com/manuals/cashregisters/Samsung1.php
Registerman
February 2012