Every communication succeeds, necessarily. You say something. I hear it, or don't. Someone else, perhaps, hears it, sort of. In any case, something happened.
Not every communication is understood. You say something. I hear it, or don't. I may understand what you said. My understanding may be all together different from your understanding, or his or her understanding. Understanding can, and often does, go astray.
But understanding is only one component of communication, albeit it important. (Needless to say, our respective understandings of understanding no doubt differ.) Understanding may be one moment within a communicative event that is itself an affective or effective machine — you understand what I say and, upon so doing, begin weeping or kissing or fleeing or punching.
But communication per se can't help but succeed. As they say, in a bit of popular phenomenological wisdom, it is what it is.