so today i decided to play minecraft today on my survival world. after a couple nights i noticed no monsters were spawning. i figured it was because i was in peaceful, since i usually play on that mode, but i was on normal. i even went to the nether and found no mobs spawning there either. i switched between modes and nothing worked. i think maybe its because I switched it from 1.12.2 to 1.14.3, but that was yesterday and mobs were spawning normally.
Turn your render distance up to a minimum of 10. Below 10, and especially significantly lower than 10, will make it seem like mobs are not spawning but in reality they are spawning the same as ever BUT are spawning in lazy chunks that prevent them from later despawning (despawning is a mob AI action and mob AI is just not processed at all in lazy chunks).