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. .
Aganju: One of the people on your level of the stairs, is a Brain Pawn (non paying member). He/she is at their 20 game limit.
Your best option would be to recruit people to join you on that set of Stairs . Perhaps post on the GO discussion board and someone may join you. http://brainking.com/en/Board?bc=118