Jack tries to place magnets on his refrigerator at home, but they won’t stick. What could be the reason?

The body of the refrigerator isn’t made of magnetic material.

Explanation:

Same reason why a magnet doesn't stick to a paper or a plastic is the reason behind why Jack's refrigerator is acting out because may be it isn't made of magnetic materials like iron, steel etc, but rather made up of non magnetic things like porcelain, wood etc.