If you keep your music all in one folder and it isn't well organized this happens a lot. I have had similar problems on multiple occasions. What I usually do is go into the settings, under album art, then deselect "download album art". The I select the "prefer downloaded art" option (this is technically applied only to the player screen but you never know) and uncheck "prefer in-folder images". After that I delete the album art cache and once the cache is cleared I will force stop the app, then restart it and go back to the settings and re-select the download album art option. This has fixed it many times for me. If you want an even better fix, go into your filesystem and delete every single album art file that isn't in an album specific folder, clear Poweramp's app data, then reinstall the app.