I've been using the same world for about 7 months now and never had an issue of mods not spawning, recently after the 1.16 nether update my friends and I created a mob farm to get more material for the nether but after we created it, it was working fine until we exited the game, the next day we joined and it wasn't working so we looked online how to fix it, as other minecraft players said if it doesn't work to switch your mode to peacful for a few seconds and then to hard mode and it should work, we tried it and once it was set to hard difficulty a few mobs spawned but after that no mobs would spawn, we have kept our world on hard diffcutly since and haven't seen a single mob spawn, we tried logging off and logging back in but it continues the same thing, we even went to the nether to see if anything would spawn in there but not a single mob in plain sight were not sure it it glitched our world or bugged please find out what is going on, thankyou