I have just noticed that when it does this one of the models states:
"fixed-effect model matrix is rank deficient so dropping 1 column / coefficient"
which explains the 0 degrees of freedom. I am not sure why my model is suddenly becoming rank deficient as I made sure the maximal model wasn't rank deficient before starting. However I think this means that problem is with the rank deficiency which I can hopefully sort. Will update if this solves it.
Thanks @nirgrahamuk for you help!