Tahm Kench bug

I am having some problems "eating" allies. I could do it at the beginning of the game just fine. However, in the later portion of the game, I noticed that I couldn't eat specific team mates. I had my ally stand right in front of me while I repeatedly attempted to use my move on him and he was untargetable. Am I misunderstanding the mechanic or is this a bug? Any help would be greatly appreciated.