unconvincing
adjective /ˌʌnkənˈvɪnsɪŋ/
  /ˌʌnkənˈvɪnsɪŋ/
- not seeming true or real; not making you believe that something is true- I find the characters in the book very unconvincing.
- His explanation was unconvincing.
- She managed a weak, unconvincing smile.