How long have you had them? When was the last time you gave them a full cleaning? Rubbing alcohol on a tissue won't get the caramelized gunk off the coil. There are several ways to clean a coil that can work well to help get the worst of the gunk off:
1. If it's not too bad, run under hot water (hold the atomizer with a pair of pliers so you don't get burned) for several minutes, periodically removing to blow through it from both ends to blow out any pooled gunk. When done, blow out and pat the outside dry. Let sit out in a well ventilated area for several minutes. Then attach to a battery and dry burn; that is, start pulsing the button for about 4-5 seconds at a stretch to let the coil get hot and burn away the gunk. Pulse for 5 seconds or so, let go for 5-10 seconds. Repeat a dozen or so times. (Note: If you are using a low resistance atomizer, reduce pulse time to 3 seconds, as they get hotter faster and can pop if you dry burn them too long.) Now rinse under warm water once more for a minute or so to rinse out the ashes created from the dry burn, blow out, pat dry, and let sit for a couple of hours. Then it is ready to vape.
2. If it's particularly bad (tight draw, little vapor or TH, bad taste) it's going to need a soak. Fill a small dish with either isopropyl alcohol, pure grain alcohol, or a mildly acidic liquid like lemon or lime juice (citric acid helps dissolve gunk), soda water (carbonation creates carbonic acid -- you can use any carbonated drink but soda water won't leave sugary residue or flavor) or vinegar (acetic acid). Drop atomizer in there for 15-30 minutes. After soaking, go to step 1.