From the Troubleshooting Guide -
Check the Obvious
Turn on the gas and listen at the burner to see if you can hear the gas arriving.
If the patio heater has an electronic ignition system make sure that there is a spark. If there is a spark but the heater will not light up, try using a lighter or burning spill to light the gas. Be ready for a flash over if you have been trying to light the gas for a while-there might be a small cloud of gas near the burner.
Start at the Bottom
The most likely reason that a gas heater will not work is that there is no gas. Take out the empty gas cylinder and replace it with a full one. Often you will feel that there is still gas in an empty cylinder but there is not enough to create the pressure needed to drive the gas burner. An empty cylinder will feel light.
Check the Gas Regulator
The gas regulator is the connection of the gas cylinder to the gas pipe that takes the gas up to the burner. Some regulators have a flip switch that prevents the valve in the top of the cylinder from being depressed-effectively an "off" switch. If there is such a switch, make sure that it is in the "on" position.
Check the Gas Pipe
Although it is most unlikely that the gas pipe can get blocked, remove the gas pipe from the regulator and from the gas inlet at the top of the heater. This pipe is pressure fitted and held in place by pipe clips. Blow air through the pipe to ensure that it is clear.
Check the Burner
The burner will be a very simple assembly. Take each part off in turn and check for any blockages. Also check that the burner apertures are set correctly. Beneath each burner is a small chamber in which air and gas are mixed. Make sure that the gas and air inlets are clear.
If Your Heater has a Pilot Light
If your heater has a pilot light, there is the possibility that the thermo couple has failed. The thermo couple is usually shaped like an inverted "v" and sits in the flame of the pilot light. The temperature acts on the thermocouple to keep the gas supply to the burner open. A good indication that the thermocouple has broken down is that the pilot light goes out when you try to turn the heater on. Thermo couples are readily available patio heater parts and usually available from stock.
If you have carried out all the checks and the heater will still not light, you can repeat all the checks more slowly and carefully or take your heater to a specialist for a service.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/steven_7f54ab9063554f66