Thanks for your helpful comments Andre. Having copied audio files from my PC I couldn't see them in Recently Added. However I've now discovered that if I rescan, then they show up after all. So my problem is solved. Thanks.
Using Windows Media Player I synced (via a usb cable) an album containing 10 tracks in wma format to Poweramp on my Nexus 6 but only the first track shows up under Recently Added. I can't find the other tracks! Help.