fitting a double dimmer light switch?

Hi,

I'm changing a standard double light switch to a double dimmer. Through the switch box I have a red, a blue and a yellow wire (and an earth into the back of the box). The existing double has an upper and a lower set of connectors with red into comms at the top and that is linked to the comms at the bottom. Blue and yellow go from L1 top and L1 bottom.

When I replicate this set up on the new dimmer switch only one of the two lights will work. I can change blue/yellow to choose which one works but cannot get both to work. I've tried various combinations but simply cannot resolve this. Can you assist?

With thanks