This can be caused by a number of reasons such as a blockage of the spark hole, gas hose, or the spark is inoperative etc., which can be tested and corrected in the following ways:
1) Try lighting the stove as usual. By using a gas igniter to set fire, if the fire is on, it means that there is gas released. It may be because the spark hole is clogged. This can be corrected by using a needle to remove any debris that is clogging the spark hole.
2) Lift the gas hob cover to see how the spark works. If it does not start a spark, it means that the spark is broken. A technician should be called for a repair.
3) If the fire on the burner is still on but at uneven power and flashes out to other areas, it means that the gas hose is not in a good condition. This is too dangerous for a self-repair. A technician should be called in for service.
Gas hobs come in different sizes. Installation guide can be studied from the User Manual.
Read more: Stove & hob buying guide