Thanks gdenby.
Assuming you are seeing the same as me, in symbols studio there is a symbol with a black square in it? This is what has me confused. The symbol was created in the "Drawing Area" layer in this document. But when I made changes to this layer any new shapes (the red circle) did not appear in the symbol but changes to existing shapes do change in the symbol.
I then tried editing one of the copies of the symbol that I dragged from symbol studio. I added the black square. This updated in all copies of the symbol including the one in symbol studio, but not the original in "Drawing Area" layer.
I've just started a new document and the symbols are working as expected but this has happened several times now in different documents.