Mobs straight up don't spawn on my world anymore. I tried uping the render distance, loading and unloading chunks. nothing has worked. It's really frustrating since i am unable to get any mob drops. Additionally, this is a problem in both the overworld and nether (i haven't been to the end yet)
Try turning on the "mob spawning" option in the cheat section.
Or use this command:
/gamerule domobspawning true