1.Generally speaking, a water heater fails to ignite when there's not enough water pressure. This is when you need to install a pressure pump.
2. If you have installed a booster pump but still notice fluctuations in water temperature or water flow, it's because the pump is not running continuously. Go and find the pressure switch of the pump, turn the adjusting screw left (counterclockwise) towards the (+) sign. Keep rotating it for half a circle until the pump runs continuously when a faucet is on and stops when the faucet is off.