I wonder, what term 詰めの甘さ could mean? The whole sentense is: 三種の器に入れてもらえなかった詰めの甘さなのだろうか
1 Answer
「[詰]{つ}め」 originally means "checkmating" in Japanese board games. From that, we often use the word to refer to the "final stage" or "final move" in all kinds of things.
「[甘]{あま}い」 here does not mean "sweet". It means "not severe", "permissive", etc. If you were being permissive or lenient at the final stage of a game of chess or something, you would not be likely to win.
「詰めの甘さ」, therefore, means "one's tendency to slack off at the end of something".