HOW are you vaping? WHAT are you vaping??
If you are vaping 6mg with an old RDA at 1, 1.2, or 1.5 ohm coil and stock (tiny) airflow, it's probably nowhere close to enough nicotine to replicate cigarettes.
IF you are however vaping 6mg w/a subohm tank, at 0.3, 0.4, 0.5, or even 1.0ohms, with the airflow toward the middle or highest setting, 6mg should be good.
If you are in the 1st category, you may either A. want to try subohm vaping (on a device capable of this, with a tank [and coil] capable of this) OR... U might just try more nicotine!
IF you are in the 2nd category, you may want to try a 9mg e-liquid (which most quality e-juice vendors WILL accomodate, specially for you. ECBlends, MtBakerVapor, etc)
Now, i simply would-not recommend 12mg if your set-up is anything like the 2nd option above. It's just too much nicotine. 12 to 18mg is perfect for the 1st option above.
back when all/most n00bs, like yourself, were ALL in the 1st option above, I was in the camp recommending 12mg, while, every1 else was recommending 18-24mg at the least.
MY reasoning, for WHY a NEW vaper should try a MEDIUM to LOW mg nicotine amount, was that, NEW vapers, will have their PV to their mouths, A-LOT... Am i correct? I usually was.

If you smoked a lot (like, every 20-30 mins, like i did/do when I smoke rather than vape...) then you're going to VAPE, A-LOT. DUH! lol. So, IF ur using the OLD style PV/COILS, 12-18mg!
However, with the introduction of these damn new-fangled "low-ohm, high watt BEGINNER devices" (which I believe have no right to be marketed as n00b devices) u need 6-9mg!
Ramsay (above me) brings forth a good point... Are you indeed getting your preferred THROAT-HIT? If not, u may want to UP your nicotine OR the amount of PG in your liquid.
For me, the whole cravings thing, stems from my cravings for THROAT HIT... Nicotine Rush is second to Throat Hit, haha. (but, my body knows when it needs more nic, too, lol)
U may want to use a higher PG (instead of higher VG) juice, if u are missing the throat hit. that cud do it... tho, u may also want to increase the nic, for better TH, as - well.
Aside from the above tips and tricks, i usually like to tell new vapers, to STOP!
.....STOP IT! Just STOP IT! Just STOP craving cigarettes, OKAY!!? JUST STOP IT! Just stop whining about craving cigarettes, and if you're still smoking 1-2, STOP IT! just STOP it!
(lol) This may sound like a bad therapist's method of getting a bed wetter to stop wetting the bed... but... I find that, it can be quite effective for ex smokers new vapers...
If you are still smoking that 1 or those 2 cigarettes a day (or 5, or 10...) and telling everyone that "oh vaping has helped me cut-down sooo much, it's sooo great!)
Then you're cheating yourself. Stop it. Stop cheating yourself. MOST of us switched to vaping seamlessly, without smoking those 1-2 a day for the 1st month or two.
In-Fact, EVERYTIME i switch back to vaping from a smoking relapse, i switch 100%, with NO cigarettes on the side. NO cigs at night before bed, or in the AM, or after meals.
JUST STOP IT! CUT IT OUT! STOP SMOKING! You're NEVER going to STOP craving cigarettes, until you STOP smoking, COMPLETELY... So STOP IT! STOP SMOKING! Duh!
You need to stop smoking, simple as that, harsh as it is, the truth hurts, eat it. Stop smoking. Every time I smoke 1-2 a day when I switch, I wind-up smoking again. STOP IT.
If you aren't going to take vaping seriously, the ciggy cravings aren't going to go away. ever. they're going to stick around and torment you for the rest of your life.
STOP SMOKING and SOLELY VAPE! If you still feel like you're craving a cigarette, after having NOT smoked, for 3-4 weeks, something is up with your brain... Trust me. lol.
You SHOULD stop craving the cigs after tht amount of time... Because.. Once you've spent that amnt of time w/o cigs, JUST vaping.. u wont WANT a cigarette..
Vaping tastes better... and when u taste a cig after having ONLY vaped for 3-4 weeks, the cigarette will taste like SH*T... Trust me... It's gross. It's like smoking on mushrooms...
It tastes like you've bundled-up a bunch of dried-up leaves, rolled them in a piece of paper, lit them on fire, and inhaled it. lol. U wont love it, trust me. So, just... Stop. K?
Hope I've helped in SOME way. &, I really do hope that you'll enjoy your new & exciting vaping journey. Have fun!