Tahm Kench can't eat Vayne ?

I just had a Normal Game where I played as Tahm Kench Support. First I thought it's just my bad skill but soon I noticed that I couldn't eat my ADC Vayne anymore. No matter how many times I tried I just couldn't use my W on her. I am pretty sure I was able to eat her in the first 5 minutes of the Game. Something must have happened after that and suddenly I couldn't eat her anymore. But I can't remember doing anything specific either which could have caused it. Did anybody else had similiar problems with Tahm ?