R Daniel Mann from Eretz Hemda answers your question at length here. He concludes it is not an issue in most cases although he does bring opinions (Machatzit Hashekel, Mishna Brura 27:15) that raise the issue of much more hair than normal and/or the tefilin sitting on so much hair that it doesn't sit on the head anymore. See there for details.
The summary of his answer is
Some poskim say that long hair is a chatzitza for tefillin. However,
regarding most cases of long hair, these opinions are difficult, and
the length is not the main issue, as we will see.
One puts the tefillin shel rosh where the hair grows (Shulchan Aruch,
Orach Chayim 27:9). As one is not required to shave his head
frequently, hair could not possibly be a chatzitza regarding tefillin. [...]
In summary, in all but the most extreme cases presented above, there
is ample reason to say that long hair does not prevent the fulfillment
of the mitzva of tefillin. We would note that many of the poskim who
raised the issue combat the phenomenon of long hair for males
(primarily?) based on other halachic, social, and philosophical
elements (see Bemareh Habazak V, 25).