There are a couple of things you can try:
- Use a higher nicotine strength, as long as you don't get any symptoms of over-consumption. The highest retail strength now is 45mg (4.5%), and there is a reason for that: some people need it in order to prevent them going back to smoking. Some ecigs don't work all that well, so you might need a higher strength to compensate.
- A percentage of smokers never stop craving a cigarette, no matter how long they have been vaping (or using any form of nicotine), and no matter what strength of e-liquid they try. For these people the only solution is to use WTA-inclusive e-liquid (it has other tobacco alkaloids as well as nicotine), or supplement vaping with Snus (Swedish pouched oral tobacco).
We don't know what percentage this applies to, it's always being argued about. Presumably they also need the other stuff in tobacco; there are multiple alkaloids including nicotine, nornicotine, anabasine, anatabine, myosmine and several others. Some people probably become dependent on yet other components in cigarette smoke, but many of those who find nicotine is not enough manage to sort it out with WTA refills or Snus. Be careful with Snus at first as some products supply plenty of nicotine (more than a cigarette in some cases), and combining it with vaping might be a little too much until you get it right.
So you could try these steps, in order:
Get a better rig (one that produces large clouds of vapor, not wimpy little wisps)
Get higher-strength refills
Try WTA liquid
Try Snus as well