quite~; pretty~; rather~; just not ~
なかなか expresses that something is more than expected (quite/pretty) or, with a negative, that it just won't happen.
In positive sentences, なかなか means 'quite' or 'pretty', often implying the degree is more than you thought. For example, if a test was easier than expected, you say なかなか簡単だった. It can also mean 'fairly' when you're not fully impressed but acknowledge a certain level.
In negative sentences, なかなか means 'just won't' or 'hardly'. It emphasizes that despite effort or waiting, the action or state does not happen easily. For instance, なかなか寝られない means you try but sleep just won't come. This usage is common in daily conversation.
Be careful: with a negative verb, it does not mean 'not very' (like あまり) but rather 'not easily' or 'not at all'. The nuance is frustration or difficulty.