Hi William,
You are probably applying too much effort. It is more tiring at first to notice more detail. I recommend having the intention to notice more detail, and then just seeing what happens.
It is interesting that you contrast following the breath vs. just observing the breath. Following the breath is also just an observation, although you have the intention to notice more detail.
You may have an expectation that you should notice more detail than your mind is currently capable of doing. There could be a feeling that “I” have to notice a large amount of detail, and this means the sense of self is involved, and this is much more tiring than doing it through an intention.
You might try just noticing the beginning of the inhale and the end of the inhale, and just “observe” the rest of the breath. When that is not too tiring try adding the beginning of the exhale and the end of the exhale. You can slowly work up to noticing more detail.
Blake