It does happen, usually for stupid reasons. I was in the same situation at PTY, watching, watching, while these guys checked their lists (usually to determine which standby's are being loaded, I wasn't SBY) and finally, they just tossed my bag on.

Sometimes bags are held off because check-in will call and say "she's just running to security now, she's been told the bag might not make it".

Sometimes it's just a bonehead doesn't load it, then the 2-3 other guys don't pay any mind to it thinking the original guy knew why it wasn't being loaded.