Like subver said, its the juice. I dont know what eliquid you use, but it happens with my NET tobaccos. I have to clean and dry burn after one tank full with those tobaccos. My coils will get gunky and dark and I can tell by the taste. I just take the rebuildable unit out, soak it in hot tap water for about 5 mins and dry burn and its ready to go again.
With my clear minty, fruit flavours this does not happen. I can refill and go for a few tanks before having to dry burn. NET tobaccos and coffees and thick really sweet flavours will do this also.
You dont need to rebuild all the time, just rinse , soak and dry burn to get more life out of your builds. Bishopheals has a great tutorial on dry burning with GG atomizers. If I can find it , I will post it.