Please can you check that your challenge settings are where you want them to be. The last dozen or so people who I've tried to challenge I have not been able as they are already playing their maximum self imposed number of concurrent games.
It works like this : Other 'Red Hot Pawn' users will only be able to challenge you when you have fewer than this number of active games.
You will still be able to initiate games yourself above this limit.
So if your challenge limit is 10 games, and you are already playing 15 or 20, nobody will be able to challenge you. This will suit some people but not all. I suspect that some people are not aware that their setting is blocking any challenges.
Or if your rating is over 1700 and you want a game, you could always challenge me !
Nick