That late finish pulled the Pilgrims a point closer to seventh as results went their way elsewhere.
However, victory - in game United dominated from start to finish - would have seen them pull level with sixth-place York City.
"It was a strange game to say the least," Cox reflected.
"It should have been wrapped up in the first 35 minutes, but if you don't put your chances away and you're not ruthless in the final third it comes back and affects you.
"We find ourselves becoming frustrated, but the lads worked hard and tried to play the game the right way.
"But sometimes you need to be more precise in the final third, and we didn't have that.
"It's taken a set-piece once again to score.
"I'm really frustrated to have that much possession (and not win), teams who get that in this league should win comfortably.
"We only had about six shots that hit the target. You've got to bust a gut to get into good areas."
United have eight games left to force their way into the top seven, beginning with Tuesday's trip to Chorley.
"There's a lot that needs doing, but saying that, results seem to be falling for us and we still find ourselves in a good position," Cox added.
"But if we're going to be more successful we need to be ruthless, and ruthless with each other."
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