What was found
CREW REPORTED- ON DEPARTURE LG AIR/GRND FAIL. LND GEAR LOCKED DOWN, RAN QRH WITH NO RESOLUTION. AIR RETURNED. WHEN PARKED FRONT NOSE STRUT FULLY COMPRESSED. CORR ACTION MX DETERMINED THAT THE NLG STRUT HAD BEEN INCORRECTLY SERVICED WITH EITHER HYDRAULIC FLUID OR NITROGEN. SERVICED NLG STRUT WITH BOTH HYDRAULIC FLUID AND NIGROGEN. NO ABNORMALITIES FOUND DURING SERVICING. OPS CHECK GOOD. CYCLED THE GEAR UP AND DOWN 3 TIMES WITHOUT ANY ISSUES. JACKED AND DOWNJACKED AIRCRAFT AND OBSERVED SMOOTH MOVEMENT OF STRUT.