No new burned items?

If by chance you took a break from lessons for a period of time, that hiccup could also cause a dearth of no burn reviews for a while. If you aren’t using it already, perhaps you should try installing the Ultimate Timeline userscript. Then you could look to see if you have burn reviews clumped together within a week span or so.

I also agree with MasterSenSenpai that there isn’t anything abnormal about your situation, unless you know for sure that you’re getting burn reviews but they aren’t registering when you get them correct. (This could be verified with the above script).