YouTube does support PMs, though I don't like using this feature since the notifications just may or may not appear. You can find this option if you go to a channel's "About" tab and look for a button with "Send message". In the same tab you can find a captcha-protected E-Mail address. You may not have noticed that before, since one has to configure those options first.
BTW, did you actually subscribe? Don't get this wrong, it's just that YouTube sometimes decides things which are hard to comprehend, so I was curious if "not increasing the subscriber counts even if there's actually a subscriber" is one of these decisions.
EDIT: I also dislike YT's PMs because you have to go to "Creator Studio" > "COMMUNITY" > "Messages" in order to reply.