Sometimes people can get stuck in their grief. I think one of the important reasons for this is when there are unresolved issues between the person who is grieving and the person who has died.
It is hard for the person to move on when there are unresolved issues that need to be dealt with first. Unfortunately as the person that needs to be talked with to resolve these issues is dead, there will never be that chance.
It then becomes an irreconcilable situation and the person who's grieving gets stuck in their grief unable to let go and move on without first resolving the issues that cannot be resolved.
(c) Baz Busbe
Regis