To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
I'm trying to challenge someone - anyone - in the Slow Go stairs (http://brainking.com/en/Stairs?tri=3477). However, there is just one name to pick for me, and if I pick him, I get the message that I cannot challenge that player because he is at his game lmit.
Is this a known lmitation? Any idea what I can do?