The reason you couldn't find John & Sarah Williams in Cambusnethan Old Churchyard is because they are not buried there. They are next door in the little Walled Burial Ground which was opened by a burial company in 1861. It would have been expensive to buy a plot there, it lies between the old churchyard and the municipal cemetery (which also dates from 1861).
John & Sarah owned plots 50 & 51 which are against the wall, so they would have been more expensive. The burial records have survived, here's the details for 50 & 51.
SARAH WILLIAMS, housewife, The Green, Wishaw. Aged 71 years, married.
Parents - Joseph Rice & Mary Stone.
interred in 50 on 1st August 1891.
ROBERT WILLIAMS, Ironmaster, The Green, Wishaw. Aged 47 years, unmarried.
parents - John Williams & Sarah Rice.
interred in 50, 26th October 1895.
JOHN WILLIAMS, Ironmaster, Wolverhampton. Aged 76 years, widower.
Parents - Robert Williams & Eleanor Williams.
Interred in 50, 18th February 1898.
MARY ANN WILLIAMS, daughter, England. Aged 19 years, unmarried.
Parents - - - - - Williams.
Interred in 51, 19th December 1870.