Many people cannot feel the breath in the nose sufficiently to keep their focus on it. This is true with many people. If you cannot feel the breath during at least 70% of the inbreath and outbreath, then you may need to change your point of focus from the inside of the nose, the tip of the nose, to somewhere else.
This video below attempts to explain what else you can do and where else you can focus during meditation. I made this video for someone in particular, but it may be helpful to many others.