The credibility of a noncredible person may increase over time if people remember the message more than the reason for discounting it in the first place. this is known as

The credibility of a noncredible person may increase over time if people remember the message more than the reason for discounting it in the first place. This is known as the sleeper effect. The sleeper effect relates to persuasion and the gradual increase of a message's effect in coordination with a discounting cue.

That is true , instead try to have a sit down talk with them