As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - FLT 8511 - AFTER TAKEOFF, GEAR HANDLE WOULD NOT MOVE TO THE 'UP' POSITION. PILOT OVERRODE THE RELASE SWITCH AND MOVED HANDLE TO 'UP' POSITION, BUT GEAR REMAINED DOWN. HANDLE WAS SELECTED DOWN. AIRCRAFT RETURNED TO BASE. RIGHT MLG SAFETY SWITCH (WEIGHT-ON-WHEELS) OUT OF RIG.