A couple of things that can happen from the factory. 1. the sock doesn't wind up around the coil to protect the cotton blanket. 2. the cotton blanket wasn't packed tight enough to keep the sock against the coil for good wicking.
A couple of things that can happen from the user. 1. Chain vaping that the wicking can't keep up with. (my no 1 problem lol) 2. not keeping the inner blanket wet. 3. juices that have lots of ingredients that caramelize.
If you wet them good and didn't chain vape too hard, I'd contact the vendor and ask for some help. Also, are the tubes (socks) black?