The gas flame will ignite, but the flame will not stay lit once the igniter is released. There are a few reasons why the flame will not stay on.
1/ Check that all the burner caps are located correctly, if they are partially misaligned or crooked the flame may not correctly connect with the flame sensor preventing it from keeping the gas tap open.
2/ Check that there is no spillage or baked on grime covering the flame sensor, this may prevent the heat from adequately reaching the flame sensor wich may require the ignitor to be held on for an excessive period before flame will stay lit
3/ There may be an issue with the flame sensor, please contact LG Customer care on 1300 LG CARE (1300 54 2273) in Australia and 0800 LG CARE (0800 54 2273) in New Zealand to arange an Approved Service Centre to attend and repair the gas cooktop.