What was found
IAH TO MAF - AFTER TAKEOFF, THE FLAP LEVER WAS MOVED TO 0 AND THE FLAPS STOPPED RETRACTING AT 6. THE QRH WAS FOLLOWED AND SUFFICIENT FUEL WAS BURNED OFF TO ALLOW FOR A LANDING BELOW STRUCTURAL LANDING WEIGHT. THE AIRCRAFT LANDED WITHOUT INCIDENT WITH THE FLAPS AT 6. IAH MAINTENANCE RESET THE FLAP SYSTEM WITH NO FAIL MESSAGES SHOWN AT THE TIME AND THE AIRCRAFT WAS FERRIED TO SDF. SDF MAINTENANCE TROUBLESHOT THE FLAP SYSTEM AND FOUND 3 FLEX SHAFTS THAT WERE BAD. THE NR 3 LEFT, NR 5 LEFT, AND THE NR 5 RIGHT SIDE FLEX SHAFTS WERE REPLACED. AFTER THE REPLACEMENT OF THE FLEX SHAFTS THE FLAPS WERE RE-RIGGED, THE AIRCRAFT WAS TEST FLOWN SUCCESSFULLY, AND RETURNED TO SERVICE.