Mini shogi: I don't understand this movement

http://www.littlegolem.net/jsp/game/game.jsp?gid=1303581&nmove=21

In this match, I needed only to put the captured pawn in front of the opponent king to give checkmate, but littlegolem did not allow me to do that move. I thought that noone could give a checkmate while drawing a piece from the outside, but in the next movement my opponent took a captured general to give me checkmate. Somebody can explain me what happened and why couldn't I give checkmate?

From the Shogi rules on Wikipedia: "2.A pawn cannot be dropped to give an immediate checkmate. However, other pieces may be dropped to give immediate checkmate, a pawn that is already on the board may be advanced to give checkmate, and a pawn may be dropped so that either it or another piece can give checkmate on a subsequent turn."

So, basically, a PAWN can't be dropped to give a direct checkmate, while OTHER pieces can.

[game;id:1303581;move:21]