At set times, some amount of decay is applied to the Comfort Level of every growing dragon that you own (excluding dragons with no happiness, ones in the Incubator with minimum happiness when incubating, or ones that are sustained by Pino's Vitamins or Jelly from Hercules Dragon). The amount of decay appears to be random, so even dragons of the same species obtained at the same time will experience a different amount of decay and consequently grow slower or faster. This makes the decrease in a dragon's Comfort bar very difficult to predict, so is best to check in on your dragons every once in a while and top up their care when possible.