Soundiiz is using the Amazon Music API provided by Amazon to read and write data in an Amazon Music user account.
While we are adding/deleting songs to an Amazon Music playlist, it's possible that these changes are not immediately applied to the playlist. That means, on Soundiiz side for example, it's possible that you are not seeing new additions to your Amazon Music playlist.
We recommend you to check directly in the Amazon Music website to see updates made to your playlists. Be aware that in the Amazon Music mobile app or desktop, you will have to refresh/clear your cache to have latest updates on your collection.
Also be aware that Amazon Music has a limit of 2500 tracks per playlist.