According to Chapter 2 "Sitting Meditation" of the book "How To Meditate" by Yuttadhammo Bhikkhu, you can try to observe movements of the abdomen, instead of nasal passage sensations:
The purpose of formal meditation is to limit our experience to the
fewest number of objects in order to allow for easy observation
without becoming overwhelmed or distracted. When sitting still, the
whole body is tranquil and the only movement is when the breath enters
and leaves the body. When the breath enters the body, there should be
a rising motion in the abdomen. When the breath leaves the body, there
should likewise be a falling motion. If the movement is not readily
apparent, you can put your hand on your abdomen until it becomes
If it is difficult to perceive the motion of the abdomen even with
your hand, you can try lying down on your back until you are able to
perceive it. Difficulty in finding the rising and falling motion of
the abdomen when sitting is generally due to mental tension and
stress; if one is patient and persistent in the practice, one’s mind
and body will begin to relax until one is able to breathe as naturally
sitting up as when lying down.
The most important thing to remember is that we are trying to observe
the breath in its natural state, rather than forcing or controlling it
in any way. In the beginning, the breath may be shallow or
uncomfortable, but once the mind begins to let go and stops trying to
control the breath, the rise and fall of the abdomen will become more
clear and allow for comfortable observation.
It is this rising and falling motion that we will use as our first
object of meditation. Once we are able to observe the motion of the
abdomen without difficulty, it will serve as a default object of
meditation for us to return to at any time.
In this guide, Thanissaro Bhikkhu wrote that you can focus on any place where the breath can be sensed, whether it is the nose, chest, abdomen or any other place:
Only when you have cleared the mind in this way, and set outside
matters aside, are you ready to focus on the breath. Bring your
attention to the sensation of breathing. Breathe in long and out long
for a couple of times, focusing on any spot in the body where the
breathing is easy to notice, and your mind feels comfortable focusing.
This could be at the nose, at the chest, at the abdomen, or any spot
at all. Stay with that spot, noticing how it feels as you breathe in
and out. Don't force the breath, or bear down too heavily with your
focus. Let the breath flow naturally, and simply keep track of how it
feels. Savor it, as if it were an exquisite sensation you wanted to
prolong. If your mind wanders off, simply bring it back. Don't get
discouraged. If it wanders 100 times, bring it back 100 times. Show it
that you mean business, and eventually it will listen to you.