another thing you might have to do if you have a digital flow controller is to turn the flow rate up on that,
when fitting a heater in a friends window cleaning van he used to work on 35-40, after fitting the heater and had the temp set to low we had to turn the flow rate upto at least 45 fro it to fire up.
this is how we did it,
have gas turned on, turn water on so it flows like you was working and then turn up the controller 1 at a time to see when it fires up, then turn water off wait for it all to cut out then turn water on and see if it fires up.
also check to see if you have the right regulator as if there is to much gas going through then this could also cause problem.