Only one part of my futon seems to be dented, what should I do?

When a futon is still new, the cotton inside is fluffy.
It gradually becomes compressed to its natural firmness.

If you sleep in the same place when the cotton is still fluffy and your body position does not change much during sleep, only the cotton in the same place may compressed and get uneven.

If this is the case, try turning the futon upside down or intentionally sleeping on a different part of the futon. It should go flat in several days.