Why does my step goal keep going back down

I'm not sure I understand what you mean.

Are you saying you achieved 20,000 steps in one day and your goal didn't go up for the following day?

First thing to check is that autogoal is turned on. 

What you should see is that the daily goal should increase by 2.5% of the number of steps that exceeded the goal so if your goal is 10,000 steps and you achieve 20,000 steps, your goal should increase by 250 steps for the following day.

(20,000 - 10,000) = 10,000

10,000 X 2.5% = 250