Water tepid from instant gas heater?

The water from our instant water heater is tepid.
We are in a soft water area - our kettle is as clean as a whistle. Gas flame seems ok water pressure is good.
Runs off bottled propane and is the only gas powered thing we have. Tried new bottle on different side of changeover valve.
It is a simple french (le Blanc) model with no electrical connection.
She don't half hate taking a cold shower!