CRJ900 FLT CREW REPORTS AUTO PILOT DOES NOT INTERCEPT LOC AFTER ARMING; CAUSING OVERSHOOT.

Narrative

MY FO WAS FLYING. WE WERE ON THE DOWNWIND FOR RWY 27L AND WERE FOLLOWING A B757 AND TOLD TO LOOK FOR THE TFC. WE SPOTTED THE TFC AND WERE GIVEN A HDG THAT WAS 90 DEGS TO THE RWY AND CLRED FOR THE VISUAL APCH. AT THAT TIME I LOOKED DOWN TO CLEAN UP THE FMS AND SET IN THE TWR FREQ. I ONLY LOOKED DOWN FOR A MIN AND WHEN I LOOKED BACK UP WE HAD PASSED RWY 27L AND WERE ABEAM RWY 27R. I CLICKED OFF THE AUTOPLT AND IMMEDIATELY TURNED TOWARD THE CORRECT COURSE AND RWY. I CONTACTED ATL TWR AND THEY SAID IT WAS NO PROB -- NEAREST TFC WAS ON THE OPPOSITE SIDE OF THE FIELD FOR RWY 26R. FOR SOME REASON; EITHER THE AUTOPLT COULD NOT CAPTURE THE COURSE WITH A 90 DEG INTERCEPT OR THE APCH MODE WAS NOT ARMED. THE FO SAYS HE ARMED THE APCH BUT WHEN I LOOKED UP AND REALIZED WE HAD OBVIOUSLY GONE THROUGH COURSE; I JUST CLICKED OFF THE AUTOPLT AND IMMEDIATELY RETURNED TO COURSE. ONCE ON COURSE; I TURNED THE AUTOPLT BACK ON AND ARMED THE APCH AND IT WORKED FINE UNTIL THE FO DISCONNECTED IT ON SHORT FINAL. SUPPLEMENTAL INFO FROM ACN 788238: WHILE I WAS PF AND CLRED FOR THE VISUAL APCH INTO ATLANTA; WHILE ON A BASE TURN TO RWY 27L; I ARMED THE APCH. THE AUTOPLT DID NOT CAPTURE THE LOC. AFTER REALIZING THE LOC DID NOT CAPTURE; THE CAPT DISCONNECTED THE AUTOPLT AND MANUALLY RE-INTERCEPTED THE LOC. AFTER THE RE-INTERCEPT; HE NOTIFIED ATL TWR ABOUT WHAT HAD HAPPENED. THEY RESPONDED NO PROB AND THE NEAREST TFC WAS ON THE OPPOSITE SIDE OF THE FIELD FOR RWY 26R.