The Collaborative International Dictionary of English v.0.48:

Unconvincing \Unconvincing\
   See convincing.

WordNet (r) 3.0 (2006):

unconvincing
    adj 1: not convincing; "unconvincing argument"; "as unconvincing
           as a forced smile" [syn: unconvincing, flimsy] [ant:
           convincing]
    2: having a probability too low to inspire belief [syn:
       improbable, unbelievable, unconvincing, unlikely]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "unconvincing":
   a bit thick, a bit thin, absurd, beyond belief, doubtable,
   doubtful, dubious, dubitable, flimsy, hard of belief,
   hard to believe, implausible, improbable, inconceivable,
   incredible, not deserving belief, open to doubt, open to suspicion,
   passing belief, preposterous, problematic, questionable,
   ridiculous, staggering belief, suspect, suspicious, tall, thick,
   thin, unbelievable, unearthly, ungodly, unimaginable,
   unsubstantial, unthinkable, unworthy of belief, weak