It's possible that we are not able to add more than 2000 tracks to your favorites songs Deezer.
That's due to Deezer, limiting itself the number of elements you can have in your favorite lists. That's why Soundiiz can't add more than 2000 tracks to your favorites Deezer.
There's also a limit of songs in a Deezer playlist, set at 2000 too.
More details can be found on Deezer page here.