I have the same issue . I have 1522 songs. Poweramp will only shuffle about 200 songs.
I have hundreds of songs that have NEVER played during shuffle.
I have tried all shuffle modes. shuffle just songs, shuffle just categories, shuffle categories and songs, etc. I have even moved ALL songs and put al 1522 song only in one folder on my phone. damonscott71@gmail.com if you have a solution