There is definitely a piece of the sentence missing. Incommunicado means that there is no way for you to communicate, or something is impeding your communication.

There's a couple of things off with the sentence though.

If the writer meant to say that the great oaks blocked their communications then it would have been better to write: Most times, one of those oaks got in the way of the transmission from the plane and they were left incommunicado.

(better be one hell-of-a HUGE oak though, or their radio signal is really paltry, because a bit of walking and you'd be out from under the tree.)