The checkbox for Synching to the left of a collection has disappeared ?

I am running LR Classic version 10.2 and I am having trouble synching to mobile and Portfolio i selected to synch one of my collections using the synch icon checkbox at the side of the collection, which worked fine at first but now I have made additions to the collection the checkbox with the synch icon is no longer available and i am unable to update my collection to my mobile or Portfoliio?
Any ideas?
