Hi Ruggy, If you already have it connected C to C and L1 + L2 try this - switch on the left hand switch so the light is on, now does the right hand switch work?? If yes the live and switched lines on the left hand switch are crossed over.
OK so the left switch is correct, but check that the live wire from the light fitting (usually red but check) goes to the C terminal and the switched wire back to the light (usually black with red sleeve but check) goes to the L2 terminal. Now link both C terminals which should always be live and on the right switch connect the two wires into the L1 and L2 terminals, it does not matter which goes in which. should work now !?



