As in... you press a key... and your main receives it.
You have 3 slaves, 2 receive the input but the third does not.
You press the key again, and now all of them have the input?

It is weird that some receive it while others do not.

I've noticed its rather easy to open a chat window on one or more clients.
And chat windows eat keystrokes...
But that's probably not your issue, if they get the command on the second press.

Perplexing.