Because the sensations at the rims of the nostrils are so subtle, there can be a tendency in meditation to breathe more forcefully in order to heighten the sensations. Try to resist this tendency, and instead allow your breath to be very light and delicate. Ideally you shouldn’t be able to hear your breathing.
(Sometimes you’ll find that you think you’re hearing the breath, but that it’s actually a purely internal sound – one that only exists in your mind. This is fine, and you shouldn’t try to get rid of that kind of sound. Instead you should be aware of it as well as the physical sensations of your breathing).
Instead of breathing more heavily, try to find the subtle sensations by allowing your mind itself to become more receptive and subtle – this is the point of this fourth stage of the practice. Making your breath coarser by snorting (yes, it can get that bad!) can make it easier to feel the breath, but rather undermines the development of a more refined perception of the breath. If you don’t manage at first to find the sensations on the rims of the nostrils, then you can be aware of the breath in your nostrils; cool on the in breath and warm on the out breath.
And sometimes people find it easier to notice the sensation on the upper lip, and that’s OK as well. Over time, try to refine your awareness so that you become aware of the most delicate sensations that you can detect – these are the true focus of this stage.
If you can find the sensations of the air flowing over the rims of your nostrils, then congratulations; now it’s time for you to refine the practice even more. For example, you can notice whether the sensations are more pronounced in the left or right nostril, and you can try to take more awareness into any “dead spots” where the sensations are lacking. Or you can become more aware of the sensations just at the fronts of the rims of your nostrils, rather than all around; just to stretch your ability to detect very subtle sensations indeed.
There are always greater degrees of refinement to which we can take our concentration.