なかなか～ない

Using the word なかなか + the negative form of a verb means that something occurred "not easily", "not readily", or just "difficult to..." It's generally used for negative situations where something you desire doesn't happen easily, or when something that's supposed to happen doesn't happen as it should.

一年生の時は漢字がなかなか覚えられませんでしたが、今は、あまり時間をかけずに覚えられます。
When I was a first year student I couldn't easily memorize kanji, but now I can memorize them without taking very much time.

もう１０年もピアノを練習しているが、なかなか上手に引けない。
I've already been practicing piano for ten years, but I still can't play it very well.

今年は十二月になってもなかなか寒くならない。地球温暖化のせいだろうか。
Even when we reach December this year it still won't get very cold. I wonder if it's due to global warming.

コンサートがなかなか始まらないので、客が怒り出した。
The concert was slow to start, so the audience broke out into a rage.