Since your battery isn't variable voltage, your options are a little bit more limited. Using darker liquids will require a lower voltage, and since yours is set to 3.7v, your resistance may need to increase on your replacement heads (using a higher ohm coil, preferably 2.0 ohms and higher). Also, from what you said, I assume you're using a fresh coil out of the box and if it's made by Kanger, there are the rare occasion that you get a bad coil or one that just doesn't work very well. You always want to make sure you purchase coil replacements from a known vendor such as Discountvapers.com.
If you're using a light colored e-juice, your ohms are higher than 2.0 and you're not taking extremely long or hard drags on your device, then the only reason I can imagine is that it's the coil you're using giving you the problems. That or it's your tank, so it'd be nice to know which tank you're using.