unconvincing

/ˌʌnkənˈvɪnsɪŋ/
adjective
  1. Not seeming true or real; failing to persuade someone that something is true or correct.
    • The actor's performance was unconvincing, and the audience didn't believe his emotions.
    • His excuse for being late was unconvincing, so the teacher asked for a note.
    • She gave an unconvincing smile, trying to hide her disappointment.