Can you elaborate? I'm struggling with quality of my employees work :/

I'm not sure what you do, but I know a ton about employees. 8-)

You get what you measure and reward.

You need written rules that the employees can follow and a way to measure how closely their work meets your requirements.

Reward those that meet your requirements, don't reward those that don't, and fire the bottom performers on a regular basis.

If something "not good" is happening that isn't covered in your requirements, adjust the requirements.

One bad egg can really drag a lot down.

Fire negative people. They're not happy anyway, and bad attitude is contagious.