I've been using PlayerPro for a long time but just recently my rating stars disappeared from my "Now playing" screen. I found an older thread describing a similar issue ( viewtopic.php?f=4&t=3305#p20565 ), but this isn't my issue; I don't see the volume bar described.
I'm on Verizon, and I first noticed this problem after they updated to Android 7.0 (Nougat). It seems like perhaps PlayerPro can't even read the ratings, because I'm unable to create a smart playlist matching, say, only five star songs. Also, if I open the "Track details" for a song that should be 4 stars, it shows as "Rating: 0". Play counts do appear in "Track details", and they are being updated. I don't know for sure that it's related to the Nougat update.
I use the "iSyncr" rating system (and have from the beginning). I just installed RocketPlayer and confirmed that it does show my ratings.