Don't blame the RSST if you aren't getting the vapor production you expect. I own three of them & they will almost choke me to death on either a VV/VW device, or any of my mechanicals.
From the sound of things, you have a poor connection on the positive pin at the bottom of the atty. They are tapered on the RSST, and if you don't adjust them properly, only the center of the pin is touching (barely). ...it will vape, but won't have any "Oomph" - you are getting voltage through the connector, but very little amperage - which is what heats the coils.
Two quick things to try; Readjust the positive pin so it protrudes below the 510 connector about 1mm (just screw it down a few turns)
and add a small Stainless steel washer or two under the positive post & the spring to help with coil tension. I would also recommend running without the filler hole plug - not really needed, and helps prevent a vacuum build up in the tank which makes a huge difference in wicking. Don't worry - it won't leak, even if you keep it lying down.
Good luck.