meaning of 詰めの甘さ


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




「[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".

