Do a .chaninfo #your_channel and see if it's +inactive, if it is.. then make it -inactive with "chanset", also you may want to see the .help chaninfo and .help chanset.
Once the game is over, the king and the pawn go back in the same box.
Might also be worth checking that the channel isn't invite only (+i), or registered users only (+R/r on some networks), or has a key (+k). You might need to remove these modes for the bot to join. Also, check that the bot isn't banned from the channel.