Silvervine is Japanese, so that's already listed; you probably tried either Tartarian honeysuckle or valerian. Immunity to all of them is pretty unusual, but it

Silvervine is Japanese, so that's already listed; you probably tried either Tartarian honeysuckle or valerian. Immunity to all of them is pretty unusual, but it's also worth noting that responses can be variable, and each one should be tested a few times spread out. (It seems like stress or uncertainty can help shut down responses. Cats are very cagey about showing inner state.)