Missing births 1827-1838

[Ira Taylor]

David Walker (c1807 Scotland) married Catherine MacAulay (c1807 Scotland) in North Uist 14 November 1827 and emigrated to Canada in 1838. They had several children in Scotland before leaving for Canada but I can't find any birth records for them anywhere in Scotland. Where they settled is in line with many other families from North Uist that lived in Cape Breton, Nova Scotia. This leads me to believe that David and Catherine had stayed in North Uist until 1838 but why can't I find any birth records of the following children?

Donald Walker 1829 Sco
Mary Walker 1829 Sco (unsure of date but there are multiple Walker twins in my family tree so this is not unlikely)
John Walker 1834 Sco
Sarah 1836 Sco
Angus Walker 1838 Sco

Any help or advice is greatly appreciated. Thanks!