Okay, now you've got me thinking. Keep in mind I'm very new at this but when you said maybe it's a problem with the "screw on bit".
Hmm, you have an ego one battery and a subtank nano? I'm not familiar with the subtank nano as I have the ego one kit...battery and tank (both ego one).
However I recall having kind of the same problem with my unit a while back. Tried a new coil, refilled juice, unscrewed everything, wiped it etc but it just wouldn't work.
I realized when I was screwing the unit back together I was do nervous of the juice flowing out that I held it upside down while screwing the battery back onto the tank...so the drip tip was at the bottom. So I tried screwing the air flow ring (?) to the tank then turning it right side up again. Then I screwed the battery back on. I figured maybe sheer gravity of me screwing the battery onto the unit while upside down was maybe causing too much pressure and it wasn't threading properly. It worked!
Then a couple of days ago I was refilling both units with juice. I have 2 ego ones. It happened again. One unit worked and the other one didn't.

I unscrewed everything and screwed it back together, being careful to make sure the battery was threading properly to the tank. Nothing. Pure frustration for sure.
I decided to interchange the tanks...it worked. Now the unit that wasn't working, is working. I'm baffled but at least it's working until I can get to the local vape shop and get them to have a look.
It's really important to have a back-up unit
@Dewal . I know the feeling of panic when your unit isn't working and the thought of a cig floats in your head.
All thus to say I'm wondering if it's a threading problem?
I sure hope you get it resolved.
Cheers