Hypothesis: Taking candid photos is harder as a professional than it is as an amateur.
Discussion: For years I have been taking candid photos, mostly at events that had many people in attendance. Nobody paid me much mind; they went about their business as if I didn't exist. Oh sure, sometimes they would notice me and give the "creeping me out" expression, but generally my camera changed nothing. But when people hire you to take candid photos, they become hypersensitive to your presence, and start performing for you. I suppose culturally we're trained to "smile for the camera", so when we know there is one around, and especially one we have paid for, we pose and smile at the mere hint of a pending shutter click. Poof, away goes the "candid".
Corollary: Candid photos can only happen when the subject is more interested in their natural activity than they are in the camera.
Discussion: Get the subject distracted with an activity. But it has to be an activity that occupies their attention to the point that they forget that there is a camera in their vicinity. I did a session with a wonderful family over the weekend, focusing on their young son. For almost an hour he did his best to smile whenever he saw my camera up, or on command, or just spontaneously. The whole time he was checking to see where I was, and what I was doing with my camera. His smiles reflected his preoccupation with the camera. It wasn't until near the end of the shoot that he forgot about me. It was the swing that did it. His dad pushed him on a swing, and was goofing around with him. Sure, it was prompting, but the son forgot that I was there and was just reacting to the swing and his dad. Finally, a genuine smile. It was worth the wait.