The answer may be that you have sediment buildup in your tank. As water heaters age, they tend to accumulate sediment and lime deposits. If the heaters are not cleaned periodically, the sediment may rise to a level that will act as a barrier between the burner and the water, making it harder to heat. An article published in a national ASPE plumbing journal states: for every half inch of sediment on the bottom of a gas fired water heater, it requires up to 70% more fuel to heat the water.