Why would some of the hairs on my head be growing slowly or not at all?

Right now the majority of the hairs on my head are about ¾ of an inch long. I shaved my head with a #1 trimmer length a few months ago. When I run my hand backwards through my hand, and then press down, I can feel the tips of hairs that are significantly shorter than the rest. Has anyone heard of anything like this happening? and what would cause such a thing? Also, I’m a twenty-one year old man and I am not taking any medication.