Mountain Biking - Valve issue on tubeless rims?

So, Ive been running fine on my crossmax xl ust rims with bonti tubeless tires. Well, wanted to run during the snow season here in wisconsin and try some different tires. Its been a while since I mounted the bontrager tires. The other set of tires I have are with tubes, so I figure, pop out the valve and core, did that. Then, got a different set of tires that are tubeless, ok, drop the valve and core back in. After struggling to get the tires on, the threaded valve unit doesnt hold air. I hold the core, screw the unit down, but it just twists, no real tightening feeling, but air is getting out? What am I missing here? Its hard enough to get the tire to initially hold air, then the valve/core leaks like nutz! Any advice. Thanks.

Time for new valve pieces.

Is it the valve core or valve stem...you must be very specific in your terminology if you want a diagnosis w/o actually bringing the patient to the doctor.

The core is the piece inside the stem...either one could leak air.

sorry about that, its not the core. It appears to be the rubber seal on the inside/underneath rim. When I tighten the nut down to the rim, I cant get it to completely snug, give the valve a push right or left and air sneaks right out. Took a look at the rubber seal underneath/inside rim connection, appears to be fine?

Are you using the o-ring that comes with the valves? The valve will not seal only from the inside. You must use the appropriate nut and o-ring on the outside to make the wheel hold air.

that was it, no O ring. Plus the seal on the inside was cracked and needed to be replaced, works fine now.