Honestly, it could be anything.
You are going from a two-hole to a single-hole carto if I read that right, true? Wouldn't you think you'd get a better draw from a 2 hole? I mean a cartotank works on a vacuum/negative pressure scheme. Going from a 2 hole to a single is going to be like turning the faucet part way off in your sink.
How dense the polyfill is in a carto can affect the draw. The polyfill material in a carto can vary from carto to carto, brand to brand. Yeah, quality control in China sucks. We've come to accept this as a part of
vaping. We spent up to $6 or more a day on cigarettes, so what's $2 on a carto? I've gotten a 5 pack of cartos that were all good cartos, but I've also had batches that 3 out of 5 were bad right from the get go. So yes, you may have gotten a bad batch.
I see no reason why the length of carto would make any difference. I will say that I only use single coil cartos. My experience with dual coils is that they will begin to taste burnt on the second or third day of use; probably because of wicking issues and the top coil burning the polyfill.
So, having said all that, I still love my cartotanks. When set up correctly they give a much better vape than a clearomizer: better flavor and more consistancy. Not one
juice delivery device is perfect and works every single time, no matter what you are using, be it a clearo, cartotank, or RBA.