Cut the mustard?

It's a sad day for Feifei as she just lost a cooking contest, and Finn tries to cheer her up by saying that sometimes we've got to accept that we just can't cut the mustard. Did Feifei miss out an ingredient?

Let´s listen the audio in this link:

http://www.bbc.co.uk/worldservice/learningenglish/language/theenglishwespeak/2013/03/130319_tews_115_cut_the_mustard.shtml

I was not very good at math, I just couldn´t cut the mustard. On the other hand I was very good at English, that´s why I´m an English teacher nowadays!

 What about you? Tell me about something you are not very good at, using the new expression you have just learned.