After performing Ghusl (ritual bath), I was drying my hair and noticed some fluff in my hair, which I may have gotten from my pillow or somewhere; I am not sure. Did this block water from reaching that part of my hair?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you doubted whether this fluff got in your hair before or after performing Ghusl, then the basic principle is that it was not in your hair before performing Ghusl. If you doubted after performing the Ghusl whether the water has reached all the parts that it must reach or not, then the basic principle is that you should ignore such doubts that occur after finishing the Ghusl. Muslim scholars underlined that doubting after having finished an act of worship does not have any effect on its validity. However, if you become certain that water has not reached a part of your body or head, then you are required to wash that part only because continuity (without interruption) is not a condition for the validity of Ghusl according to the majority of the scholars.

Allah knows best.