Okay, I've kind of got it under control now. I've been skimpy with refilling my carts. I really don't want to run out of liquid, so I do as little as possible to saturate the filler. Not good. I noticed that my pre-filled carts are drenched in the stuff from top to bottom, and they don't have this problem nearly as often. I'm going to be more generous with my juice, even if it means running out.
So yeah. If you still haven't figure it out yourself, make sure your refilling technique is up to par. Putting a drop at the top of the cart will give you 1-3 drags if you're lucky. fill'er up (but don't over-fill). If you have the problem even with a full cart, then drip. I don't know why dripping works so well in comparison to pushing the atty into liquid-soaked filler, but it always gets it going when all else fails.
I think the atomizer may be heating up its end of the cartridge-filler way too much & way too fast, drying it out while not really drawing much from the rest of the cart. That's the only explanation I can come up with.
It's a weird problem. Every time I think I've got it figured out, it happens again unexpectedly. Dripping is my go-to solution right now, and the ability to drip safely is one of the strengths of the 510 anyway.
one more thing- i've noticed that twisting the cart while it's attached will soak up the atty a bit and can resolve the problem temporarily. This is bad because when you pull off the cart, a lot of the small fibers from the filler may be stuck to the atty. But it seems to work for me so far. (update: and it's still working.. not a single burnt drag after a simple twist of the cart, and it might be helping vapor production, cause I've been making clouds. i'd love to hear if this works for anyone else.)