It'll be the juice gunking up the coil, no amount of super expensive cotton will magically make it work significantly better (although some might work a touch longer).... IMHO.
Might i suggest grabbing a second tank so you can alternate between the two, and not be hitting one as frequently. I hit mine pretty heavy (i can appreciate the frustration when builds don't last), and having two vapes to alternate between definitely helps, i can easily overdo it when i keep chugging on the vape, even with very forgiving juices.
Thinner juices will help to, if you can stick to 50/50 juices it'll wick faster and keep up with heavier use better.
That said, how's the wick in the middle when you're getting in there for a rebuild? If it's black and/or burnt looking in the middle, you might just be hitting it too often and not giving the wick enough time to soak up juice.