Early on in Beetlejuice, Adam and Barbara perform some scare tactics - a beheading and a gruesome face removal. No one notices.
Shortly after they go to see Juno (their 'caseworker'), who says:
It obviously doesn't do any good to pull your heads off in front of people if they can't see you.
Later they go to see Juno again and show her their grotesque monster faces. She is pleased and praises them for their ability to be scary.
What has changed? How did they suddenly gain full "ghost powers"?
Despite doing essentially the same thing as they did in the beginning - for which they were admonished - there is now at least the presumption that they'll be able to scare the Deetzes.
Unless I missed the explanation, this seems like a pretty big plot hole.