Can't notice the breath
Hmmmm... interesting. Try this: do controlled breathing for a dozen breaths (count breaths on fingers and do a moderately-slow deliberate breath-ins and breath-outs), then for a dozen breaths let the breathing happen on auto-pilot (keep counting on fingers. I assume you have twelve fingers LOL).

When you breathe deliberately you can't possibly NOT notice the breath, right? After all, at that time you are deliberately using the muscles of the chest and diaphragm to suck air into your lungs. So you should feel yourself pulling pulling pulling not too fast, slowing down, then stopping, holding a little, then going down and out out out not too fast, slowing down again, a tiny pause. It is what you yourself are doing, so you can't miss it, right?

Then when you let it happen naturally, the same attention you used for deliberate breathing should continue on to natural breathing.

Also, do you sit in Lotus / Half-lotus? Try sitting in Vajrasana - it arches your back a little more and forces to open the chest and diaphragm. Perhaps (I don't really know but I'm guessing) your breathing is too shallow because you are in the habit of blocking your lower abdomen? Vajrasana is a good posture for this kind of practice, to penetrate the lower chakras.