Obviously, a person cannot be forced to act as an executor, or to be a guardian. Perhaps their circumstances have changed since the will was made, and of course in the case of a guardian, the children in question may have reached adulthood.
You should check regularly that the people who may have a role to play after your death are still willing and able to do so. This is a sensible thing to do so especially if they:
fall ill or become disabled; or
become a carer for someone else.
When a person does not want their inheritance
It may happen that a person named in your will does not wish to accept the legacy you intended for them.
They may wish to receive nothing at all from the will; in which case they can disclaim the gift.