On your phone, go to Settings > Applications or Apps > iReal Pro > Storage > tap ' Clear Cache' and reboot your device.
Also make sure that you're logged in to GooglePlay with the primary account for your device.
If this doesn't solve the issue, uninstall iReal Pro, reboot your device, reinstall iReal Pro and reboot your device again. 

IMPORTANT: Back up your songs before you uninstall the app. You can download it again for free from the Google Play store as long as you use the email address you used for the original purchase.
Here is how to make a backup of your songs.