No offense, and you probably dont mean it to come across this way, but that actually sounds passive aggressive, like you are trying to guilt them. How would you feel if your kid had a party and for whatever reason someone was left off the guest list (intentional or not) and they approached you like that.
Really, offense is taken. Casually mentioning it to them might have resulted in the totally plausible explanation that Mylitta suggested. Mylitta explanation makes me feel better about the situation and I am grateful for forums like this as the perspectives of others can really help put the situation in perspective. I am not trying to guilt anyone.

If my son was invited around several times to a friends' house for playdates and he regularly played with that friend after school in the playground and that friend was one of a group of three friends that regularly played together and had done so all year, then there is no way I would leave that child off an invite list. It just wouldn't happen. I would not allow it. There has been no falling out as far as I know.