Observers and dispensers don't work for some reason

I tried to make multiple mob farms for example this one: https://www.youtube.com/watch?v=guvMmdeZqiI

I made it exactly like the one in the video and i checked it multiple times, but the observers and the dispensers don't do anything. I have no idea what is going on, but it is not how it's supposed to be working... It would be really nice if it could be fixed!