What likely and actually often happens is the flavors you have simply won't vape well at higher wattages with what you are using for RDAs. This can get to you with regular replaceable coils as well as with something rebuildable where certain flavors at say 20w just won't cut it regardless of what you have attached to the battery or mod in use. You'll think something needs replacing or rebuilding when trying out a few flavors and then you hit one with everything still the same that you can turn up to 40w!
Now for tankjs that range about 20w for the max like a Nautilus you will see the same type of problem where one flavor can cranked way up and vape like a champ and then the next you have to keep down at 6-7w! And it isn't always the wattage at fault either. I have Japanese cotton here I could actually toss since some flavors simply won't make the grade while they suffice on an Atlantis when turned up. The funny thing is when you see the volts that's due to changing the display mode not the wattage to voltage since there is not voltage mode.
Somehow you hit something to be seeing the display change over. In the case of the wattage mode only most of the new mods seen now will simply adjust the voltage for you automatically according to the resistance. With one mod here with the Atlantis on I will see 3.8v when turned up to 30w., 3.1v at 20w, and for 22w it with show 3.3v. For the iStick 20w = 3.4v, 22w = 3.6v and for 30w where I have the other set still having an Atlantis on with the 0.5ohm coils that will be 4.2v. So far I haven't seen the display change at all from having the wattage in large characters and pressing to hold both plus and minus buttons will either lock or unlock the present setting.