Hi. This sort of problem is rare on newer models, so it is unlikely to be drawer wear. First thing to try is to turn the power off at the wall, and then try closing it. If it stays closed, then you have an electronic problem.
Otherwise, it can often just be something stuck in the latch at the back of the drawer. If you leave the drawer open, then make a sale, most machines should make a 'CLUNK' sound in the drawer. This is the latch pulling in to release the drawer.
I have found a little video clip about fixing drawers. It is a different drawer from yours, but it shows the latch assembly, and what to look for:
http://www.learn-cash-registers.com/cash-register-drawer.html
Go to the bottom of the page, and you'll find the link to the video. Once you click and open the video, be patient while it loads.
Hope this helps,
Mark
Mark, July 2008