AN AIRBUS A-320 ACFT RETURNED TO FIELD AFTER LEFT MLG WOULD NOT RETRACT. GEAR PIN NOT REMOVED PRIOR TO DISPATCH; AFTER MAINT WORK.

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I ASSUMED ANOTHER MECH STOWED DOWN-LOCKS. (SYS/EQUIP NOT DEACTIVATED/REACTIVATED.) WORK INCLUDING MULTIPLE TIRE CHANGES ON ACFT X DURING WHICH I CHANGED TIRES AND OTHER TECHS HELPED WITH PAPERWORK AND GEAR PIN R/R. I CONTINUED ON TO ACFT Y TO CHANGE BRAKE ASSEMBLY #2 AFTER WHICH I CLRED UP AREA AND NOTICED TAGS WERE TAKEN TO BE FILLED OUT BY OTHER MECHS SO I ASSUMED HE AGAIN STOWED DOWN-LOCKS. I DID LOOK AT THE L MAIN LNDG GEAR AND SAW NO STREAMER HANGING (PRE-DAWN FROM APPROX 30 FT AWAY). AFTER TKOF L MAIN LNDG GEAR DID NOT RETRACT WHEN COMMANDED. FLT CREW DID GO AROUND AND LANDED; THEN RETURNED TO GATE WHERE L MAIN LNDG GEAR DOWN-LOCK WAS REMOVED AND STOWED. NEVER ASSUME SOMEONE ELSE DID PART OF YOUR TASK EVEN IF THEY HAD JUST DONE SO PRIOR TO THE EVENT. ALWAYS CONFIRM EVERY STEP OF MAINT PROC THAT YOU SIGN OFF AS COMPLETE. BE AWARE OF CONSEQUENCES OF SUCH ACTION/INACTION AND THE EFFECTS ON AIRLINE OPS AND PAX SAFETY AND CONVENIENCE. IF INSTALLING A DOWN-LOCK MAKE SURE IT HAS A STREAMER HANGING FROM IT; AS AIRBUSES ARE ALMOST THE SAME SIZE AS THE ACTUATOR ROD THEY COVER.