tall pull out larder wont work properly?

Hi,
Kitchen ahs been recently fitted but the 300mm wide tall larder wont sit in its housing properly and wont pull out smoothly.
I was there when the fitter fitted it and we both agreed we couldnt see why it wasnt working smoothly.
It was centered to the unit and without the racking in each runner was smooth and nice. The units and level and plumb but still wont work.
Its getting a little annoying now as neither the fitter or myself know what to do.
Can anyone help.

Thanks