Limited Amount of Lords in a Castle?
Solution 1:
Circumventing this limit is rather tedious and requires you to manually add more spots for the lords to stand in in the castle/city halls' configuration files. The lords stand at pre-determined spots, and when there aren't enough spots left, the rest of the lords are omitted from showing up. Exiting and re-entering the town shuffles which lords show up on visually on the feast, though it would take quite some exiting/reentering to greet every single lord in attendance.
Some castles and cities do have larger halls, allowing more lords to visibly attend. Which ones are the larger ones I am unsure, but the Nord have the largest I can recall at least.