As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - ON FLIGHT 4272 (DEN-BOI), AFTER TAKEOFF AND DURING CLIMB, THE SLATS WOULD NOT RETRACT (AT 9000 FEET) AND A SLAT FAIL CAUTION MESSAGE WAS RECEIVED. RETURNED TO THE FIELD AND ACCOMPLISHED A UNEVENTFUL LANDING. MAINTENANCE REPLACED THE BPSU. OPERATIONAL CHECKS GOOD AIRCRAFT RETURNED TO SERVICE.