When they left for college then later on to start work, it was easy to notice those days in which a child would go quiet for a while. I learned that this was a normal process in adapting to the new environment(s). I would hardly be settled if they didn't pick my calls or respond to my texts; calling them continuously didn't help, either.
Letting them be at times worked and over time, my husband and I realized that silence did not necessarily mean that they did not want to talk to us, after all. Did you always notice it when they became distant or it (at times) hit you that it had been a while since you last spoke?