Birding is chancy.
When I take a second loop around a trail or make a repeat pass at a spot visited earlier, it amazes me the things that I saw the first time that I did not see the second time (sometimes the second pass is better, and I feel lucky to have repeated my steps).
Birding is just like that. To connect with a bird, you have to be going in a direction that puts you in the path of a bird. Slightly different trajectories could cause the connection to be missed. For all the sightings I do make, there are probably many more that do not happen because a difference of a few feet of distance, a timing off by a few seconds or minutes, or a noise that calls my attention to the left but distracts me from the right. That birding is chancy is not a principle of birding that I stick by, but more a key understanding of how birding, and perhaps life, works.
I have been deepening this understanding since I started looking at birds, and even using that phrase, "birding is chancy," for years. Two recent experiences from the last couple weeks in the Bronx demonstrate the understanding perfectly.