Depending on why a flight is payload optimized, many times it won't be until the last minute until we get approval to put more on. Gate agents are initially assigned a passenger count limit that they can go up to. Once all of those passengers and all bags are scanned on, those numbers must be sent in. Only at that point can the captain call dispatch, who then contacts load control, who then sends in the final number that can be boarded based on all of the most current conditions.
As you can imagine, there can be a lot of issues stemming from this process. The first problem is a lack of understanding of how it works, both from flight crews and even some newer dispatchers, to especially many of the gate agents. Many see that initial number and think its a hard count that can't be changed. Sometimes it's a grind to get them to understand even after explaining what needs to be done. Combined that with the time pressure that naturally happens near the end of the boarding process and you can probably see what happens many times.
Of course, this all relies on proper communication. The dispatcher has no idea anyone is getting left behind until the pilot calls them. The pilot has no idea anyone is getting left behind until the gate agents tells them (ha!) unless they specifically ask, and even then it's a hope they are given correct information. Heck, sometimes the flight plan paperwork doesn't even list the flight as payload optimized if it was printed before the flight was labeled as one.
It's a very clunky process that leads to a lot of headaches. I wish it wasn't like this, but it is what it is until someone with a much higher job title decides to change it, which I don't see happening due to costs. Hopefully they can improve it at least, given the increase of payload optimized flights with the new passenger weights.
