Robertson is a very common surname in Fife, as you know.
In my tree, I have a fisherman Andrew Robertson, born 28 July 1704 in Buckhaven, the old fishing town near Wemyss on the Firth of Forth. His wife was Christian Sibbald Logie, born 23 Jul 1699 in Wemyss. But their son, John Robertson, was born in 19 July 1730 in Wemyss.
I am descended from James Logie Robertson, a salter, born 31 March 1732 in Buckhaven.
Do you have a death record or other documents for your John Robertson?