The breath may be hard to find for different reasons, but the main one seems to be a lack of sharpness/mindfulness. If you then search for the breath and hold it forcefully (not mindfully) the mind will indeed dull down, because it requires energy. I find it best in such moments just to sit and wait. Let whatever is in the present moment come into awareness, mainly other feelings in the body and sounds. This resting will give the mind more energy fueling the mindfulness, which then will have an easier job finding the breath. In fact, often the breath comes by itself, you'll be surprised.
Or so it happens to me.