Phone charger only works at certain angles?

The charger for my mum's Nokia Lumia 635 now only works at certain angles. She's been using the same charger for about 8 months, but for a couple of days or so it only works at certain angles. She said it only works when the top of the phone is facing one of the walls in her bedroom and when the phone is lying down on the bed next to her. It won't work if she actually has the phone in her hands.

Why does her charger only work in certain angles? Does this mean her charger is about to die?

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