Some meditators who are not familiar with the Mahasi method report having difficulty observing the abdominal movements.
Actually, the movement of the abdomen is a large object compared to the tip of the nostril. If we relax and breath from the diaphragm and not the chest, the abdominal movement will be clear. It is a physical manifestation of the element of motion that is to be felt rather than merely noted or observed.
If it is not clear initially, take just one deep breath or hold the breath for a few seconds, then breathe normally.
As concentration develops, the abdominal movement becomes more distinct, whereas the breath at the nostrils becomes more subtle.
The abdominal movement is just one object among many that should be observed. Pay attention to the sitting posture and touching sensations too — this will help to ground your awareness in the body.
Always precede a session of sitting meditation with one of walking meditation. In walking meditation there are many objects to note, and it is difficult to walk really slowly if the mind is wandering. Walking very slowly and paying attention to the details movements of the feet will quickly dispel any wandering thoughts.
When you go to sit after at least twenty or thirty minutes of walking meditation, the fog of thoughts that obscures the abdominal movements will have disappeared.
Walking meditation is in no way inferior to sitting. It was with the support of intensive walking meditation that Venerable Ānanda attained to Arahantship in the early morning after walking for the entire night.
Venerable Moggallāna practised walking meditation relentlessly for seven days from the time of his ordination before taking up sitting practice. Due to the powerful mindfulness cultivated during walking practice he was able to attain Arahantship in a short time after being admonished by the Buddha.
Mindfulness of respiration or the touch of the breath can be useful at times when the mind is very wild and distracted, or tired and confused. However, if the mind is calm and serene you should have no difficulty focusing on the abdominal movements — as long as you patiently note the wandering thoughts and other secondary objects they will gradually fade away and the abdominal movements will be clear.