You need almost 1 gpm of steady water flow to keep the burner on and heating the water. You may just be on the edge- it starts but then turns off so you get a slug of cold water then turns back on.
Make sure that you only turn on the hot faucet and turn it on all of the way. You set the water temp on the Girard display not by mixing cold water. If you are not getting enough flow to the shower with the handle set all of the way hot, then unscrew the shower head and look inside the base for a white button with three holes. These are put there to conserve water but sometimes act too well and don't allow enough flow. Remove the button and screw the shower head back on.