lettuce leaves

the outer leaves on my lettuce are starting to get a sort of mottled brown look to them any ideas what it is ??

cheers

 

I have some cos lettuce growing - majority is doing really well, but I do have three that are doing the same thing, one is almost flattened. I can only think that it has to do with moisture - as the brown leaves are also very soggy and the whole plant seems to have 'flopped'..