Mastery never seems to go down

I keep studying topics on college algebra and testing well the day I study them, then completely failing out 2 days later.  If I get the questions right ONCE, my course level mastery level bumps up, and never comes back down.

It's just signaling that if you do something right once and then never again in your life, you're done.  The mastery level is just showing me the result of cramming, not responding to a lack of retention.

The mastery points for a given topic do seem to go down, but the course mastery has hit 84% once and it's not coming back down even when I bomb out of the course challenge.