First thought that comes to mind is you probably have an airlock in your piping system – probably where your pipe drops below the outlet level.
Raise the pipe work so that it has a slight but continual fall from the tank outlet to the water heater inlet level.
Next point of concern is did you put any wrapping around the pipe when you buried it cause if you did not you are sure to get condensation forming within the interior of the fuel line which will cause you grief in the future.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians