Tubeless set up - valve stem leaking

Make the hole in the rim tape with a circular/conical thing, then heat up the end of the valve stem before pushing it through the hole. Be careful not to overheat it - hand hot is plenty.

It helps the rim tape to stretch rather than tear and you get a better fit around the valve stem once it cools.

What is this o-ring of which you speak?  Not seen one on the 3 sets of hunt valves I own. 
Also this may be obvious but some of the bases of the valve stem are flat or ovalised and if that's the case the long axis of the oval has to be paralell to the rim wall - it lets the valve sit lower in the rim centre channel. 

I had this until i tightened up the valve ring with pliers. Could be a problem if I need to fit a tube on a ride.

Might not be the valve seal. Could be leaking through spoke holes into the wheel cavity then out of the valve hole.

I agree with henrymorcs - sealant should solve small issues, though re-taping the wheel is worthwhile if it's a bigger leak.

I had a very similar issue back in May and adding sealant sorted the issue. I was quite confident the rim was taped properly - although I could hear the air leaking near the stem it was a relatively slow leak (25 mins or so to deflate) and so easy enough for the sealant to sort out. If it's losing pressure really rapidly it may be best to double check the taping as it's more hassle to sort after the sealant has gone in. I found putting a tube in and inflating it to force the rim tape fully down was quite helpful although I know some tubeless tyres are hard enough to get on the rim without tube so it may not be worth the thumb pain...

For context: the wheel arrived fully set up tubeless and - in theory - ready to go. However, after repeated loss of pressure (with sealant inside), I took the tyre off to find out that the rim tape was poorly set up and so the system was leaking air. I applied some new rim tape and set everything up to find the tyre slowly leaking (without sealant), and reluctantly decided to go ahead and add the sealant. Thankfully I've had no issues since - hopefully you'll have the same experience. 

Have you fitted the O ring with the lock ring? Don't over tighten.
Have you filled with sealant or is this before filling? Filling may seal it.

Most likely the tubeless tape has got a bit damaged where the valve goes through - always use a pointy thing rather than a knife to make a small hole so that it doesn't tear when you push the valve through it. I've also had issues with overtightening the valve nut and with a damaged o-ring - it's better for it to be just finger tight and it may be worth swapping the o-ring if it's not sealing well.

So, I'd be looking at re-taping the wheel and maybe replacing the valve nut o-ring if it's damaged/cracked.