Unable to sustain mindfulness on subtle breath I noticed in my practice that I was unable to sustain my attention on breath when it became relatively subtle . Is this because my mindfulness is not mature enough to hold on to subtle objects of meditation? What sort of action could help me to overcome this?
There is nothing wrong with losing your focus of the breath, as long as wherever the mind goes, you are aware of that moment. That is, in my view, perfect practice. You are not restricting yourself to the breath or anything. Which is correct! Wherever the mind goes, it goes... you can't control it. So by simply watching where the attention of the mind falls upon, you are fully focused in the moment. Just like watching a butterfly gently landing onto different flowers, being carried by the wind. We have no idea how strong the wind will blow nor do will know in which direction... but, you can still watch the butterfly attentively, as it lands.

But that's just my view. My understanding of correct practice.
Metta.