You can't expect every post to get upvotes unless you get on some autovoter trails, then they become more guaranteed unless the service goes down or the voters decide to pull you off from their list.

For myself for instance I've noticed that some of those voters only vote 50% compared to 100% on my 2nd post of the day. It'd be way more rewarding for me if I posted daily or for instance put this one on a schedule and let it post tomorrow instead of having days where I go 10 days with no post at all.

If however you aren't on any autovotes yet than it's quite normal not to get any votes on some posts, it shouldn't discourage you. You should look into how much rewards you earn on average after a certain period rather than looking into each post individually.