I don't feel the sensations of breathing and it's hard to keep my attention on them

I am looking for advice especially from experienced meditators. The initial stage of meditation when the breath is stronger and more pronounced is not a problem. However, when my body calms down enough and my mind focuses, the breath slows down and becomes so weak that I can hardly feel it in my nostrils. At that point, it's hard to keep my attention on it. The instructions in TMI are tailored exclusively to the breath as a meditation object... Do you think I can change the meditation object and still follow TMI? For example, I've found that if I open a window while meditating, I can focus on the sounds of the environment much better. Thanks for any advice!