Spigot is, by default, tweaked to improve performance. You or your players may notice that monsters, villagers and other mobs don't move at a distance. You can change the following settings which will restore those distances to near vanilla settings:
I also like to change the following settings to change the block and exp orb grouping:
You should experiment with these settings taking into account the performance hit in your specific environment.
entity-activation-range: animals: 64 monsters: 64 misc: 32 tick-inactive-villagers: true entity-tracking-range: players: 144 animals: 48 monsters: 64 misc: 32 other: 64
merge-radius: exp: 1.0 item: 0.5