I made a 3 by 3 portal with End Portal blocks and eye of the Ender in each block but it doesn’t work. I did this many times in Creative mode, with and without a friend
The eyes have to face the inside of the portal.
You can do that by inserting the eyes while standing in the center of the portal, just make sure you get out before adding last eye, or you will finish the portal and immediately activate it, sending yourself into a boss fight.
Some sources say that only the last placed eye has to face inwards, I am not sure if that's true or not. It looks better if they all face inwards anyway :P