why did the canterville ghost lose confidence in his ability to frighten people?

The Canterville Ghost lost confidence in his ability to frighten people because the Otis family were immune to his supernatural onslaughts such that he began to doubt his own prowess. He grew weak and tired, the terrible excitement of the last four weeks had begun to take its toll on his such that his nerves were completely shattered and he started at the slightest noise. For five days he kept to his room, making up his mind to give up re-embellishing the blood stain on the library floor. The Otis family did not want it which meant they clearly did not deserve it. To the ghost, they were people evidently on a low, material plane of existence, and quite incapable of appreciating the symbolic value of sensuous phenomena.