DURING APPROACH TO CHARLOTTE, THE MAIN GEAR WOULD NOT EXTEND. THE FLT DIVERTED TO GREENSBORO. THE RIGHT GEAR EVENTUALLY EXTENDED AFTER APPLYING EXTENSIVE G-FORCES, BUT THE LEFT GEAR WOULD NOT. AFTER LANDING, A 29-INCH WHEEL CHOCK WAS FOUND WEDGED BETWEEN THE RADIUS OF THE LEFT WHEEL WELL AND THE INBOARD WHEEL WHICH PREVENTED THE LEFT GEAR FROM FALLING DOWN. THE AIRPLANE HAD BEEN POSITIONED SEVERAL TIMES THE NIGHT BEFORE, AND THE MECHANICS RECALLED PLACING THE CHOCKS IN THE WHEEL WELL. AN A-CHECK, AND A WALK-AROUND INSPECTION, HAD BEEN COMPLETED PRIOR TO THE FLIGHT'S DEPARTURE FROM WASHINGTON. EACH INSPECTION REQUIRED AN INSPECTION OF THE WHEEL WELL AREA. THE FIRST OFFICER STATED THAT HE DIDN'T SEE THE CHOCK WHICH WAS MARKED WITH YELLOW REFLECTIVE TAPE. A FLASHLIGHT WAS USED DURING THE INSPECTION.