Here's my theory, from experience.
A good friend of mine is opening shop soon here in Los Angeles (Cerritos, CA to be exact). He has literally thousands of Boge & SmokTek cartos. We've been using these two brands of cartos for a while now, and with regards to Smok, their entire line. There was a time when I even swore off Boges (blasphemy, I know) in favor of the then-new 1.5ohm DCC's - they were just so awesome.
A month or so ago, I remember I did the usual punch a new carto for my tank. It leaked. I'm pretty OCD with punching my cartos: check the fill and punch on the thicker side, 3/4 turn from when the punch punctures the tube, etc. I pretty much had a 100% success rate with carto tanks up until that carto, so I thought meh, just bad luck. Next one, leak. Next one, same. I went through about 12 cartos, different boxes/wrapping color, & leaks. This is with 40/60 VG/PG KBV juice, btw, which is already pretty thick.
A few days ago, we got in a new batch of Smok cartos. Kid you not: we opened a few boxes and looked at the filler, and it looks terrible. You can acutally see the "roll" of the filler, like a rolled up carpet. It's no longer "tight", and yes, I have come to find that Smoks are typically looser than Boges, but these are just sloppy. None of these cartos are appropriate for tanks, let alone actually vaping.
We then look at all the flanged cartos, the ones that are meant for carto tanks like the one Madvapes & CCV sell. Conspiracy! These are tight, and the filler looks amazingly uniform. No leaks, perfect draw.
I'm of course joking (kind of) about the conspiracy angle, but there is definitely QC issues at Smok. When I get home I'll post a picture - the fillers are seriously terrible, uneven, loose, sloppy. I know in this infancy of vaping there is always the next big thing...before Boge was the only answer with cartos, then Smok came along and are really picking up steam and gaining a huge following. I bet that they are just hammered with more orders than they can fill and QC is suffering.