I'm playing Minecraft and there's a HUGE lack of enemies

So, I have been a mine craft player for a few years now. Lately when I play though, it seems like zombies and stuff are almost NEVER spawning. I know that there are some enemies spawning, because when I go mining, I will VERY occasionally hear a skeleton rattle, or a zombie moan. But it is so rare that I see a mob like this that I'm not having any fun playing the game.

I also remember that when I played MC before there were WAY more mobs that would spawn. Does anyone know why there would be a scarcity of enemies to fight?

Just some Info, I'm currently playing on normal, and playing on hard doesn't fix my lack of mobs problem. Also, It is so severe that I can stand on the surface in the open, and ill be lucky to have a lonely zombie spawn and attack.

Are you standing somewhere dark? Don't forget that mobs only spawn in the dark.