Hair growth glitch

Also do I have to shave everything rather than just shaving it to a certain length ? thanks for your help.

The feature has been glitchy from the start.

i would suggest you post about the issue on ea answer hq forum where you can alert them about the glitch also tagging @EA_Cade

Hair growth is really slow.
I'm not sure if it's a glitch or not.
There is a bug thread on AHQ here where they are looking for save files from people who play without mods and experienced this, maybe you could send yours?
https://answers.ea.com/t5/Bug-Reports/NEEDS-SAVES-HSY-Facial-and-Body-hair-don-t-grow-according-to/td-p/11685124

To your other question, there is a choice on the sink or shower or whatever you're using to either shave entirely or shave to preferred length. Preferred length is the style you set in CAS.