The Cambuslang OPRs are quite good, from 1746 there is a very good marriage register, nice condition and easy to read. The hand changes in January 1798 and becomes a scrawl but improves by November 1817 and ends at April 1820. Proclamations of Banns from May 1820, good condition, end at December 1854.
So the period you are looking for is covered and it may be that either your ancestors were married in another parish or that they did not pay to have the Banns recorded or, they were married in a church other than the parish church. One of the Secession churches perhaps?