Builders Warehouse gas heater leaving us in the cold 🥶

We bought a gas heater Builders Warehouse Sunward and it has been giving us problems since day 1 and when we returned it they told us it had “air” they advise us to open our gas and let it run out for few minutes which we did and the problem didn’t go away ,we’ve put in a new gas and it still takes over 30mins for us to be able to switch it on and when it on it switch off on its own,we are cold yet we have a heater with full gas .