Is the power rating of the sauna heater sufficient for the size of your steam room? Check the product page or the user manual.
Did you take the uninsulated walls and glass surfaces into account when calculating the cubic size of the steam room?
Are the heating elements functioning properly? To find out, remove some stones from the sauna heater and check if the heating elements warm up when the heater is switched on. If a heating element does not turn red, it is damaged and needs to be replaced before further use.