Why are some iron objects magnetic and some others not magnetic?

It all depends on the way in which the iron's 'domains' are aligned. Iron is inherently magnetic as a material, but can become demagnetized as time goes on, due to sudden impact or when affected by another magnetic source. For iron to become magnetized 'domains' need to be in parallel with each other so the magnetic field created all flows in the same direction. An unmagnetized iron source has all of its domains misaligned A brief explanation of domains are a series of positive and negative charged atomic particles within the iron itself which behaves much like tiny magnets themselves.