Mobs not spawning - Nether and Spawners

On Better Together:

My spawners stopped working. I have 4 spawners: Spider, skeleton, zombie, and blaze. Yesterday, the spawners were working normally. But, the next day, it seems the spawners are broken, and won't spawn anything.

Then, I noticed mobs aren't spawning in the Nether. After I killed all mobs in the nether fortress, there are no mobs spawning there.

This issue is occurring on a synced world.

So, I am wondering if there is a problem with mobs spawning in the game..