You know, as a grandmother I've had some of these things happen:
1. birthday of grandchild #5
Went to water park. Was trying hard to photograph both #4 and #5. Almost all pictures of #4 turned out really, really cute. Almost all pictures of #5 sadly did not. Dilemma. Do I show Mommy all the pictures?
I didn't. I showed her a few of both and that was that.
2. grandchildren 1,2,3
Similar situations. Sometimes one child just is "easier" to capture on a certain day. Sometimes I'll take pictures of all three together and one will have decided at the precise moment the shutter snaps to stick his finger in his nose, thus making the whole picture look unpleasant. Do I keep the pictures? Yes. Do I show their Mom? No.
They may have actually taken a couple of pictures with your son in them that turned out to be less than flattering. It may be that he wasn't even the ONE that made the pictures less desirable, but because of something these were not pictures the daycare chose to pass on.
One day? I'd let it go.
If they go for a week without any pictures of your child? Yes. I'd inquire.
I fear that if too many mothers complain about one day without a picture of their child, then the daycare will stop doing it altogether.
What you MIGHT do is if for example, there are one or more pictures of kids stringing beads, asking him if he strung beads that day and see what his own answer is. It may be "Yeah, Mommy!! I'm making you a bracelet but it isn't finished yet and it's a surprise!" or it may be "No, I didn't really feel like stringing beads so I got a truck and went over to another part of the room and played with that all by myself."