In the battle against Surge (round 2), my rental Gengar used Hypnosis 20 times against Tangela, and they all failed. I did not know that certain types of Pokémon were immune to Hypnosis, however, I am assuming this must be the case. I believe Hypnosis normally has a 60% accuracy, so the odds that all 20 times would fail would be 0.0000001%, and so I do not believe that was what happened.
Tangela did not have any status ailment (no paralysis, etc), was not confused or anything like that.
Why is Tangela immune?
By the way, the reason the Tangela could not take out the Gengar was because all its moves were normal type (thus not working on ghost), except for Mega Drain, which only had 5 PP and was not very effective.