Advice on filling a empty gun off divers tank

[angrybear]

Don't quite understand when you say gauge on tank not showing, but something worked when filling ?
is there a gauge or manometer on the rifle or is it only the cylinder gauge ?
if there's a gauge on the rifle just go by that, until you replace the cylinder gauge.

If not just be methodical, reconnect the rifle, close the cylinder bleed valve, slowly crack open the main valve while you watch the gauge,
if you hear the rifle valve open & hear air flowing but the gauge does not show, then stop, you need to replace the cyl gauge before doing anything else simple as that.

Remember of course that the cyl gauge registers against the outlet back pressure in the hose (unless you have a dual gauge regulator fitted to the outlet)