From time to time you may get a DUD that has a burnt taste from the start no matter how well primed. Chunk it, you cant save it. If your draw become too tight it is ready to be replaced. Also, if you have a really good eliquid and the flavor is too muted go ahead replace just to enjoy the flavor. Vendor liquids are often high in flavor percentages and will clog a cart fairly quickly. I have been DIYing all of this year and don't have that problem nearly as often. If you don't DIY I would suggest you look into it. It is much more satisfying and will save you tons of money. Also, if your focus is on mixing you wont be so apt to try all the new gear coming out. Find the APV and tank system you like and stick with it. If you want a cart to last for several months you can vape unflavored liquid. It's the flavoring the gunks a cart not the PG VG or Nicotine. I make mine 60/40 PG/VG with 18mg Nic. Doing that I used two carts in eight months. The only reason they finally died was because I filled them with some flavored liquid. Unflavored has a natural sweetness and taste close to a analog because of lack of flavor. It is better than you might think. Also, I recommend vaping at 60/40 is make the liquid a little thicker and avoid flooding issues. I seemed to have much more flooding when I vaped 70/30. The best carts I have found are the Boge 2hole single coil 2.0 ohms non-flanged. Out of a 75 I only had 2 duds. See if you can find Buckys Bargain Vaping on Facebook for the best deals. Hope this helps and good luck