Command Blocks will have trouble finding entities in some worlds. It happens randomly, and I just experienced it in my command testing world. Tried teleporting some creepers to me, even though not entity was found. I tried some other mobs, like endermites, and it still wasn't working. Also, yes I did try using chat, typing /tp @e[type=creeper] Gwobble_Gwobble, and that worked. I even reloged, still had the problem. Also, I found that some worlds are effected by this and some aren't.