I recently played a game of Mahjong on the internet, and I had gotten this hand
[2 Characters] [3 Characters] [North Wind] [North Wind]
[4 Characters] <- Drawn Tile
[Melded Pon of 2 Bamboo] [Melded Chi of 3-5 Bamboo] [Melded Kan of 1 Characters]
but it wouldn't let me declare Tsumo. I had a few possible reasons why I couldn't but I don't know the game that well. Can you not have a pair of Winds to complete your hand? Or is my hand invalid in some way?
Here is my discards, it might serve some purpose -
[Characters] 7 [Bamboo] 4, 3, 1 [Dragons] Red, Green, White [Winds] East, West