You must be able to concentrate in order to receive hypnosis.
When the hypnotherapist gives the instruction, your mind must be able to follow. For eg, imagine yourself walking in a quiet nature. If you cannot concentrate, your mind can be either blank or drifting to some other images that have no relevance to what is being told.
There is nothing wrong with mind drifting. It is just that you may not be able to achieve the desired effect of hypnosis.