What was found
DURING APPROACH, WHILE SELECTING FLAPS 15 TO FLAPS 20, ONE OF THE FLAP INDICATORS STAYED AT 15 FOR ABOUT 3 MINUTES AND THEN SNAPPED TO 20, NO ASSYMETRY FELT IN FLIGHT CONTROLS. DECLARED AN EMERGENCY AND DID A GO-AROUND. FLAP INDICATOR WORKED NORMALLY WHEN FINAL FLAPS 25 WAS SELECTED. THE AIRCRAFT WAS GROUNDED. MAINTENANCE REMOVED AND REPLACED FSEU-1 M10331 OPS CHECKED NORMAL, IAW FIM 27-51-00-815-812 TASK 22, AND AMM 27-51-01, OK FOR SERVICE.