Dave Root made a reaction save from Mick Danzey early in the first half, but this was the only chance United made in a poor opening half.
By the interval K's led 1-0 when on 29 minutes a shot by Luckett deflected off Steve Shea. Gary Phillips did well to get a hand to the ball but full-back Danny Brooker slotted home the ensuing rebound.
United greatly improved in the second half and camped themselves in K's territory for long periods. The Ducks pressure and build up play couldn't be faulted but they seemed to lose their way within 20 yards of the goal, although Root was called on to make three good saves.
Against the run of play K's went two up after 73 minutes when Phil Wingfield cut inside and shot low past Phillips.
Late on, the crossbar foiled Danzey's chip over Root and Gavin's close range follow up into the roof of the net was ruled out for an apparent hand ball. It was one of those days.