I have the subvod original and the mega tc. Both have quirks about them that really get on my nerves. On the mega, the button light will flash for 2 reasons: 1: is that the battery is dead, which you know and 2: either the coil has a short, the ohms are too low/ too high, or the 510 connection is not touching the connection in the battery. I was using a coil from Atom Vapes made for kanger tanks and it is ever so slightly taller than kangers. It had pushed the tanks positive pin out a bit further than the kanger coils and when I screwed it into the battery section it squished the buttery connector pin down below the rubber insulator. I couldn't figure out what went wrong for a long time. Out of the 3 kanger tanks I own, only the tank with the longer Atom Vapes coil would work on the mega tc battery. I took some angled tweezers from the coil master build kit and very gently worked the battery connection up to where it was above the rubber insulator. That fixed my problem! The coils you used may vary in length and you could possibly have the same problem i had. Make sure it's off and check that positive pin in your battery. If it looks like it's sunk down in the insulator your mod will not fire. I hope this long ass explanation helps man!
PS. Another word of advice is to consider getting a box mod. I use my subvods every now and then but you get way better performance out of a regulated box mod. You can adjust your watts, a lot of them now days are small and pocket friendly, and the best part is they have spring loaded 510 connectors so it resolves this stupid issue altogether!