Well, one bad mod doesn't spoil a whole batch. There are mistakes made on EVERY high-volume assembly line. Many users report (including me) perfect function from Smok or Wotofo products (made by the same manufacturer, along with a couple much smaller mod brands)

Most likely a simple fix, such as no battery connection/looseness in the battery bay, or poor 510 soldering....one good drop could cause either one to become a critical issue.