Why is there a zombie that simply refuses to attack me?

I'm in a single-player world on my laptop. Here are the settings of the world:

• Game Mode (Survival)
• Difficulty Level (Normal)
• Cheats (Off)

So, it's nighttime and I go out to kill mobs, but I notice that there's this one Zombie that doesn't attack me. I kill all the mobs around it, and this lone Zombie stands there just staring at me. I'm thinking, "Oh, maybe it didn't notice me or something," so I prance over to him and smack him with my fist. The Zombie bounces back hurt, but does not move or make any motion to attack me. I repeat this action again and again, until the poor Zombie falls over red and I get a few orbs of XP.

Is this just a glitchy mob? Or is there something wrong with my game?

I would appreciate a complete and truthful answer. Thank you for taking your time to read my post.

This could be a problem with your computer. As @creulcat said, Please include more information in your question.

This might work, but I don't know about you:

• Option 1: Please turn off your computer, leave it to cool off, turn it on, open Minecraft, comment if it worked.
• Option 2: If Option 1 didn't work, try this: Launch Minecraft with more RAM, comment if this worked. Example: If your PC has 4 Gigabytes of RAM, then launch Minecraft with 2 GB of RAM.
• Option 3: If Option 2 didn't work, close all apps, Yes ALL of them. even File Explorer, and then run ONLY Minecraft, then load your world. Comment if it worked.

Information: Minecraft runs at 20 ticks per second, if your PC is unable to keep up with this speed, things will be slower in the game. Like blocks will break after 1-5 seconds, Mobs will respond after some time, You know what I mean.

Source: Minecraft Wiki - http://minecraft.gamepedia.com/Tick - "Minecraft's game loop normally runs at a fixed rate of 20 ticks per second, so one tick happens every 0.05 seconds. An in-game day lasts exactly 24000 ticks, or 20 minutes.

However, if the computer is unable to keep up with this speed, there will be fewer game ticks per unit time. As the vast majority of actions are timed based on tick count rather than on wall clock time, this means that many things will take longer on a slower computer."

Strange, as zombies even walk through sunlight to get to you. Sometimes when a zombie is going for me and i hit him he briefly pauses for a moment. But after 1 or 2 seconds he continues to chase me. Aren't you mistaking villagers for zombies?

Zombies do despawn if you move far away from them. That's not a glitch, but zombies being AFK is.

Also, Zombies don't spawn in peaceful mode so that won't be a problem.