Unfinished business in life, strong emotions tied to a person that's still living or a property that belonged to the deceased, basically the "wait but I have one last thing to do" effect. I and the rest of the ferrywomen can talk to an Earthbound spirit as much as we can, tell them that they're dead and paradise is a nice place so they have nothing to be afraid of, but if they're absolutely not ready to go even after talking to them, we--as non humans--can't interfere with their doings.

[She pauses, looking off to the side as though thinking of something else.]

There's also the...angry or unpleasant ones. We're taught to use our spirit energy to shield ourselves just so an angry ghost won't be able to harm us. The emotions of a human are very strong indeed, some have even caused curses upon the living.