Hi Steve,
My Dad does the same but only with certain foods. When he is given sandwiches, he lines them up neatly and doesn't appear to know what to do with them, it's like he has forgotten - I guess he has. He's ok with other food but did go through a phase of not eating very well until he was prescribed something for depression.
I imagine you have tried all different foods and can only suggest you might try 'fun foods' (strips of raw carrot, cheese, cooked chicken etc) that are also good for your Mum and hope that, after playing, she'll eventually try eating them! If she's ok with drinks maybe you could try millkshakes, hot chocolate and fruit juices, at least she will be getting some nourishment that way. I expect someone here will point you in the direction of a fact sheet and have more ideas.
Best wishes,