Protanks are inconsistent in how long the thread is, and where the airhole is - it causes trouble SOMETIMES with ProVari.
I had this discussion with ProVape before I bought my ProVari - because I like Protanks!
I got lucky - all my ProTanks work great, and I don't have a "gap" OR any airflow trouble.
Since you got the ProTank from them, you should send them an email, and ask for advice or to swap for a different one.  Many people find they have to loosen a ProTank JUST A LITTLE to get airflow - but it shouldn't have to get wobbly!!
It's probably an airflow issue.  
You also may want to check these other couple tips once you have airflow:
 - what's the "Ao" reading - the Atomizer Ohms?  If you have a 2.4 ohm or higher coil, then 3.6 may still be a bit low!  You can turn it up a bit (I vape 2.4 ohm coils at 4 or a little above all the time).
 - Another ProTank tip - if you get "dry hits" - a little burning - especially if it's NOT the first draw but after - then your ProTank might not be letting enough 
juice get to the coil.  I regularly pull the little airtube off the coil, and carefully remove ONE STRAND of the wicks there.  It lets the coil get wetter - but not "flood" - and lets the ProTank work much better.  It may not be needed if you're not getting dry hits - but I found it works great for me.  It also may depend on how thick your 
juice is  - mine's usually about 50% VG.