Mob farms not very effective in 1.12

I've been trying to create a mob farm for some time now, but they never seem to really work in 1.12. We're many with this issue.
Normally we only have 1-2 mobs drop every 10 minutes which isn't very much.

I've tried to create the simple mob farm in this link: https://youtu.be/VZgA8Kp8b0M

This was found on Minecraft wiki page: http://minecraft.gamepedia.com/Tutorials/Mob_grinder

Is this normal or is there something wrong with the way mobs spawn in 1.12 in farms?

Update: We have now tested about 5 different kinds of mob farms at different levels in a normal single player world and we're still not able to get mobs to spawn naturally in them.