I had the same issue when I first got my UFS. Take the whole pin assembly and plastic piece out. Take it all apart. There should be 3 o-rings, 1 larger and 2 tiny orings. Clean, inspect, and lube the orings. Start with the pin, put on the larger o-ring, then the flat metal nut, and then the 2 tiny o-rings. After you push those to the base of the pin, put on the plastic piece. You may have to work it past the tiny o-rings. Screw the plastic piece and the flat metal nut hand tight as hard as you can without stripping. There should now be resistance on the pin, but it should still slide up and down. Now screw the whole unit into the UFS and you should be good to go.
This is how I fixed mine. I was missing one of the tiny o-rings and it was in the wrong place when it arrived to me, so only by thoroughly breaking it down and reassembling it properly was I able to make the leak stop.
those instructions copied and pasted from inquisitorj