Something I've noticed I do and I'm curious if others have the same issue: Sometimes when I'm walking, someone who is walking in the other direction will say hello. I'm almost always lost in my own thoughts (I get particularly involved in them when I'm walking because walking is boring), so it takes me awhile to "wake up" from my reflective state and re-gain awareness of my surroundings, including the ability to respond to my surroundings in a timely manner. I will say hello back, but because it takes me so long to bring my mind back to the outside world, the person will already have walked past me by the time I say it.
Also, I have a minor voice modulation problem. My voice is soft and it takes some effort to make it loud enough for others to hear me. I have to consciously think about speaking loudly enough sometimes. So, when I'm walking and meet someone, my brain's energy is being used to recognize that someone is talking to me and slowly getting my mouth to work to respond, and I'm not concentrating on speaking loudly enough, so my "hello" is very soft and probably inaudible to the person who greets me.