My storage heaters are not charging?

My flat has unidare storage heaters model WMS712N. All but one were working nicely on an overnight charge. Now all of them have decided not to charge up. I am not doing anything different. Input setting is 4. Might be older model as input and output dials have slots. I am due to have electric meters changed would that have anything to do with their demise. It's getting a bit nippy. Will speak to landlord. Just wanted to check problem out first.