Commonly, this is caused by some residue accumulating on the burner near the spark electrode. This creates a layer that prevents the electrode from sparking properly, causing the clicking but no ignition. If the burner heads are thoroughly cleaned, does it improve the ability for the electrodes to ignite the gas?
The next step would be inspecting the wires for any bare spots or spots where the insulation is thin. Another thing worth checking is identifying if there is anything on the electrode tip where it bends to arc to the burner head.