This is actually an issue with air-hole placement on the bases of many (but not all) Protanks bases ...and, now apparently, Unitanks as well. The air-holes are (supposed to be) drilled just at the top of the 510 threading, but loose manufacturing tolerances sometimes have these drilled too low so that, when screwed onto a device, the air flow is actually cut completely off (especially on devices that have no air channels cut into the top of the 510 threading). There is really no way to correct this without drilling larger (and higher) holes.
There are, however, a couple of workarounds:
1) Get an
airflow controller - this little device allows you to raise the tank up and provide the necessary spacing needed to get air flow ...the same thing you are doing by unscrewing the tank. It's not 100% effective, depending on how low your holes have been mis-drilled, but works most of the time.
2) Use EVOD (or, in your case, Smoktech) coil heads - these heads, because they are meant to be used on smaller eGo devices, have smaller base diameters and will allow air to draw up from the bottom even if the Protank base airflow holes are closed. Many people went to this solution simply to get a tighter, less airy, draw on Protanks than with the Protank heads due to the same principle.