is it an age thing? I've got a Juvie Rotkeil, had him/her about two months & it has shown no interest in my plants: Java fern & Vallis, not much on the menu I know, but still doesn't touch them? Are they on Borrowed time?
Ditto silk or real tough plastic type plants, severums are devils for just ripping plants to bits, I cant even keep anubias or java fern with mine they just destroy it.
I had success with a large Java Fern but it died off after a while. Not sure if it was Lilly or not though...
Valis gets mown down and also dont think if you have success with one plant at one time that it will work again later... I had a form of Hygrophelia over take my tank while lilly was in it. I tried it a few months ago and she just destroyed it.