Is the blue light on ? If it is, there is a bill acceptor tilt. How the machine responds to the tilt depends on the menu setting 3.3.3 (Bill Tilts). Your machine is probably set to ignore it, but there is a problem. Check all the wiring and connections, including where the sentry bezel harness passes near the door hinge. Be sure there's no damage.
If the blue light is off, no tilts, and the bill acceptor just ignores bills, check the menu to verify the bill validator is installed (3.3.1). Check the bill/voucher limit to be sure it's not set to 0 (3.1.4).