oh I see the problem now.
If the player placed the last card from the hand to column, then the game would automatically reveal the column.
But if the player merged from 2 columns into 1, then the game would ask if the player wanted to reveal the column. I was lucky that this was the case and my opponent selected "No". Once the player chose "No", even in the following turn, he could not reveal the column so that I had 2 extra turns to finish the game.