The problem as i read it is that you dont get juice flow to the coil although the cart has sufficient; this problem can occur with dripping too, and it's not because there is not enough juice but that the juice is not getting to the heater coil ...
A certan draw force (strength of inhale) can be enough to activate the coil (light comes on and coil heats up) but not enough to draw droplets from the metal wicking mesh to the coil. That's what I think is happening.
So, when you notice vapor trail off or burnt smell or running dry, try a more forceful inhale, even if briefly, followed by your usual inhale. You might well hear the short strong inhale pull some juice onto the coil. This is what is sometimes referred to as a 'primer puff'; for it to solve this particular issue it needs to be strong (usually by primer puff is meant pre-heating the coil, but in this case it is getting droplets of juice to the coil that is the aim).
I find I need to do this every so often (about every third puff) with my 901s and then all is well.
Hope this helps
Let us know if it does.