While igniting, there is an override that allows propane to flow even without a flame.
Once ignited, there is a thermocouple that must heat up or the water heater assumes the flame went out and shuts off the propane. (eventually, it tries to ignite again repeating the process)
1) Is the thermocouple in the flame? If it got bent to the side, it may not be heating up.
2) Is the thermocouple gone bad?
3) Is the circuit board failed, so even though the thermocouple says there is flame, it still shuts off the propane once the ignition cycle is complete.