Most songs have difficult and easy phrases. Sometimes people will practice only the easy parts and convince themselves that they have “practiced”. However, I would go so far as to say that the easy parts need no practice at all. (Of course just because a phrase is easy for you doesn't make it any good musically – often we have to go back over the easy phrases to make them into truly deep expressions.)
Of course, there are also many people who concentrate on the difficult phrases by themselves. They practice these difficult parts over and over again to perfection, only to find, when they try playing the whole song, that they still stumble over the phrase in question in context. We can always say that such a player just needs to practice more, but this isn’t very enlightening. Let us say instead that this kind of practice, in and of itself, is mistaken.
Merely practicing a difficult phrase over and over again is not a good idea because it fails to take into account the phrase preceding it. The finger position and the kari/meri neck position will invariably have an effect on the next phrase. If you fail to take this into account, then all the practice you put into your difficult phrase could come to nothing when played in context, or during a performance.
Therefore,try practicing difficult phrases from the preceeding phrase, even if it is very easy. Indeed, the seed of the trouble you are experiencing with the difficult phrase could very well lie within the easy phrase before it. Once you discover this, your practice can become much more efficient.