He knows what shake and lie down mean. But sometimes he just pretends he doesn't get it. What should I do?

Practice with him more and be consistent. Is there anyone else giving him commands? Are they the same commands that you're giving him?

You're not the pack leader, that's why he won't listen.

Buy Cesar Milans book, he's got great tips and they really work. Or watch his show or clips.

His pack leader way of training really works.

Good luck.

Other people in my family give him commands, but they are the same commands.

My boy does all the commands I have taught him but sometimes gets confused when my partner asks him to do them...

I taught him "up" which is where he balances on his back legs.
He responds better to hand actions than words as when I taught him each command I did a different action for each one.

I noticed when my partner does it he uses the actions slightly differently.
This may be why.
It also could be because he knows I am the 'leader' and my partner is more of his 'play buddy'

I agree with the assumption that your sweet doggy may not know that this is a command and not a suggestion. A dog owner can sometimes send mixed signals to their dogs by letting them be the leader in some areas but then expects them to obey others. Not that this is truly the case with you but believe me I still struggle at times with my 9yr old zu lol he is very stubborn!
All it takes is one slip up of me letting him do something he's not normally allowed to do or even a family member allowing the same and that's it he thinks it's a free ticket to do whatever he pleases for the next day or two. Good luck and yes working consistently every day with positive reinforcement will work wonders!