What gauge and what kind of wire are you using?
Also your Mech may be due for a cleaning, take it apart and clean all the threads, contact points, and the switch mechanism, dirt and grime can rob battery power and keep the full voltage from getting to your coils, you need a clean and well adjusted Mech to maximize performance.
Some mechs are better than others in this regard, my manhattan clone is SS hits noticeably softer than any of my other mech tubes with the same battery and atty, had to be due to design or materials, just doesn’t have as much conductivity and has greater voltage drop than say my broadside or smpl mod.
Sounds like a little too much wire mass may be the culprit as well depending on the wire size and type used.
I prefer 2.5mm on my single battery mechs, I get a more responsive vape and less ramp up time. Also, prefer 4/5 wraps over 6/7, again for faster ramp up and a more powerful hit.
My go to simple build for my single battery mechs currently is 4/5 wraps of 24ga Ni80 over a 2.5mm bit, comes out to .17, might be lower than you are comfortable with as its right at the amp limit of most batteries, no issues with mine as I use quality batteries and I know what to look for, if you are unsure of your batteries amp rating I suggest doing some research and getting some Sony, Samsung, or LG batteries for your tube.
You could always use the same build as above with 26ga Ni80 or Kanthal for a little extra resistance to play it safe.
Keep the wire mass low and keep the builds simple and small for best results on a single battery tube Mech.