The sensations of the incoming and outgoing breath become finer and finer, threadlike, as you deepen your focus upon the infinitely nuanced vibrations that play in the central core of the body, especially in the spine.
The subtle sensations of breath may, at first, not be noticed because of the habit patterns of mind that are conditioned to chase after distractions of thoughts, memories, desires, likes/dislikes, pain/pleasure, etc.
You have the capacity to notice and feel exquisitely subtle sensations created by the movements of breath. I believe it just requires getting past the gatekeeper of the analytical mind just long enough to simply feel in the moment…and then slightly deeper.
The “slightly deeper” part, although not difficult to access, can be where the mind gets in the way. Even with strong intention to feel into subtle currents of the Life Force, the mind may not truly recognize the invitation of the moment when it has the opportunity to go once again “slightly deeper.”
The mind, in normal waking state, that vibrates in resonance with Earth density, cannot help but continue in its constant engagement with body, mind, ego, desires, etc. Recognition of the subtle Life Force seems to happen in those fleeting gaps when the mind is not “firing off,” in moments when the feeling connection is made with the sublime vibration (Kundalini). In those moments, there is only sensation/vibration, with an awareness of a deeper underlying connection with a nectar-like sweetness.