Hi Aileen
Only about a third of events were entered in the Parish Registers around this time, before the start of official registration in Scotland from 1855. It may be that the family had moved away from that particular area after George was born and records for elsewhere were not so good. Also, registers have been lost and destroyed over time.
Always hard to tell why there are gaps in the registers, ie, why you can find one birth but not others, as records in this period were patchy in general.
Monica