Sometimes the little thermocouples can get a build-up of grease/matter (food), often this can look quite clean, however, please try and scrub a little further with a scotch brite, you can be a little abrasive whilst keeping them straight.
Please could you also double-check that the flame is touching the thermocouple, if this is not touching, the safety sensor will kick in and stop the gas supply = no flame.
Once you have completed these steps and if the issue continues, please complete the Warranty Form and attach a video of the cooktop in action.
Please note: Warranties do not cover maintenance, installation or user errors as per our Warranty T's & C's, fee's will be on-charged.