If iCloud Sync stops working or becomes slow, the data in the cloud may have become corrupted and will need to be deleted.
On your Mac in iReal Pro > Settings, turn off iCloud sync.
Your Mac, Apple menu > System Settings (Preferences), click on your [Username, Apple ID] > iCloud then Account storage > Manage button. Wait for your apps using iCloud to load.
Click on iReal Pro in the apps list, click Delete from iCloud.
This will delete the data in the cloud but not on your device. Wait at least 15-20 minutes to make sure that it gets deleted from the cloud.
(If iReal Pro is not listing, then the data has been deleted by another device or you have not turned on iCloud sync within iReal Pro on your Mac.)
You can now go back to iReal Pro and enable iCloud Sync again. (We suggest doing a backup before turning it on.)
 
If you have a number of devices using iReal Pro iCloud sync, enable iCloud sync on one device, wait a few minutes for it to establish a connection to your iCloud and progress through the sync process before turning iCloud sync on in another device's iReal Pro.
 
Suggestion for multiple devices: choose (or create) one that has the most up to date library with all your songs and playlists and enable iCloud Sync. We then recommend deleting all content from the other devices before enabling iCloud Sync. If you do not do this, enabling iCloud Sync will merge the data on your device with the data in iCloud which can sometimes cause duplicates that will need to be removed manually.
(You might like to do a backup of all devices before doing this just to make sure you have all your existing songs.)
 
Always allow some time for changes to sync between devices.