Why don't my ratings increase

Well if you get a 1 or a 2 that is a significant outlier from a 4.79 and would most definitely cause your rating to drop. While 4s and 5s would even out to a 4.5 naturally, keeping your score stagnant. It Uber's rating scale fair, far from it, but mathematically this does make sense.

If you haven't done 500 rides, you'll find your rating jumps around more than if you have.  If you're a new driver, Uber and Lyft both pretty much ignore your ratings for the first 100 trips unless they receive negative comments in which case they'll send you an automated message informing you of some things you could "improve".  

Once you've completed 500 rides, even a 1 won't have a huge impact on your overall score, but it's also slower to increase your rating to a higher level.

It takes a while to go up or down Fuxchu because your rating is an average of your last 500 rated trips. Hang in there - in the long run - your average should stay solid, one or two bad ratings will not hurt you! Good Luck!