Is it leaking at the base of the stem outside the wheel where the nut is? If so, your tape may have lifted in one spot? Happened to me on a set of Assaults I had and it drove me nuts trying to figure it out. If there's any spot where the tubeless tape contacts the rim that is either uneven or not properly prepped/cleaned, the air will escape under the tape, find its way through the nearest spoke access hole along the rim channel, and finally out through the point of least resistance which is the base of the valve stem (outside of the rim). The o-ring that goes under the nut is not there to keep air in, its there to keep the stem from rattling and keep dirt out. I could be wrong but it happened to me and lets just say I wasn't a happy camper because I actually bought wheels that said "tubeless ready" on the side of them. Good luck with them and I hope my experience helps you find the leak.