Different tanks = different wire/coil clamp methods = slightly different techniques
Tanks like True you can mount like Doggy - wrapped around screw (most popular)
but had a washer/clamp method/option too - but found it was a bit wanked plus sat coil a bit too low
Others have an eyelet mounting - perfectly suited if using half wrap coils, with posts on same side of jet/nozzle
where the coil, 6.5 wraps just simply inserts no worries - but pita if using a 6 or 7 wrap
Others have a slot mounting, either vertical or horizontal, but even this can be a bitch on some tanks
The Adventa Manta MTL was a sod on thin wires, often the wire would fight and slip out from under grub screw
think that was best suited for slightly thicker flat wire coils, but was a sod for my 29awg std kanthal
The True, wrapping wire under the pan head screw is quite easy to do once you got it sussed
If you still can't get on with it - the Hastur 3.5ml has gotta be one of the easiest tank to mount a coil in - either way round
the slots seem to allow a coil to mount at a slight angle, straighten up and legs slip into the slots
the position of grub screws in relation to slots themselves seem to be positioned just right and grab the wire without fuss
(unlike the Manta MTL which was a real battle to stop the wire fighting to jump out as you tighten - jeez what a pita)
Think end of the day, it is a case of you just adjusting/mastering the slight technique or qwirks of each tank
and finally settling on which tank you get on with that works for you
I'm sure you'll suss it real soon