The cCells are finicking and do not work well with too high VG... or with liquids with lots of sweeteners, they gum up fairly fast.
I've had some that lasted me a few weeks, others a few days.
For any pre-build coils, I find that going at the max wattage never works well, I always keep a good 10W lower than it's max rated for the maximum.
BTW, higher VG does not mean better flavour, just bigger clouds and a tad sweeter (and a LOT more steeping). PG is the better flavour carrier. The high VG is more a fad towards cloud chasing, unless you are sensitive to PG.
Note that if you have too high nicotine (as sub-ohm does increase the intake almost double) which might be what's causing you headaches.
You could try adding 1-2 drops of distilled water in your tank to help the fluidity.
That said, they actually are my favourite coil cartridges to rebuild, as they are wonderfully built, easy to take apart and rebuild with whatever coil and cotton.
And because if how they are, my guess is that they are simply not primed right. You don't close the airflow and sit as that actually prevents the seeping/wicking effect.
You need to prime them as per any other coil... 6-8 drops in the chimney done slowly to let the liquid be sucked into the ceramic, then using your bottle tip, fill the side holes with a bit of pressure, a little bit at a time holding it sideways, rotate and do same for each of the three holes, until you can see the liquid pearling on the other holes.
Then put it in the tank, let it sit for 20 seconds, close the airflow and suck on it without activating the mod for about 3-4 gentle pulls. This should help saturate the ceramic better, but don't overdo it, you don't want to flood it.
Open the airflow, give it another 20 seconds, while you can flip it upside down for a few seconds then repeat (if you overdid it and flooded it, liquid may come out from your tip).
This should do it.
As you appear to have the newer SS ones, those are 40-75 W.
Try at 40W and increase up a little at a time but I suggest that you don't go at over 65W.
Maxing the wattage will only decrease it's life.
If it's still not working for you, you might have a dud.
I do note that you should use these with Temp. control... you should ensure that you are setting it correctly, wattage at around 50 and temp. starting at around 420F and increase one setting at a time until you find the right balance.
I know that some mods when new can be a bit stupid and need to be turned off and on and go through all the modes until it starts activating correctly and recognize the coils and starts doing the power output correctly. Note that you should be sure of which mode that you're on. One type of coil on the wrong mode (NI coil on TI mode will not work properly at all, it'll barely warm up, for example)