There is apparently a bug in site programming that causes an occasional error of 1 in game number count. If you "send feedback" (link at bottom of every page) the admin people should fix it for you.
Go to the tab at the top of the page for "My Settings" and then to "My Challenge Settings". You will see a box giving your maximum number of games for challenges. Yours is probably set at 1, which means that no-one can challenge you if you already have that number of games in progress. Set it to a higher number.