Help! Lightroom classic sync issues

Hi!

I am suddenly unable to photos to my 'all synced photographs' catalog? Previously (for the last 14k images...) I have manually dragged from 'all photographs' to 'all synced photgraphs' to give me access across all web and mobile devices - but for some reason I am no longer able to do so? 

I am really hoping someone can help - it is driving me daft! 

thanks in advance! 

Dougie