That usually indicates an "over-
juice" condition of one form or another - topping off with too much, overfilled pre-filled carts, overfilling yourself, flooded atomizer, etc.
It takes some time to find the right level.
Take a read
through the Joye 510 subforum, if you haven't already... lots of good stuff over there!
Joye 510 - e-cigarette-forum.com • The place for electronic cigarette reviews, news and chat
EDIT: Sorry... forgot to add how to get the right level (doh!)...
If the liquid is pooling at the top of the cart, it's too much - you want the filling to be visibly moist, but with no puddles.
I've also had this happen when there wasn't enough filling in the cartridge, in which case either a little less juice or a little more stuffing (if you're refilling your carts) should do the trick.
Tim