That portion of the game relies heavily on surprising the enemy, so if you try to do the same thing a couple times in a row, it will fail. It tracks both the last move you made in general, and the last way you moved against the city (including the one in which Sathra died). It also relies a bit on chance, and if you did not destroy a certain dam earlier in the game, you will be in trouble regardless (the game is a bit evil, especially on the main paths).
You'll want to do an assassination or light some barracks on fire to get them to expect something else before doing the raid. You'll also need more than 60 soldiers left to have a chance at such a raid (and it does come down to chance).