Hi, I've checked the names index for both 1851 and 1861, they do not seem to be in Glasgow when census was taken.
Was Isabella's maiden name Rankin? (Looks like this family travelled around quite a bit)
If so, they may have been in Edinburgh in 1851, since a child was born there in 1854. Another child born Glasgow in 1857, then another born 1862 in Dunfermline, so quite possibly the family are in Dunfermline in 1861.
You could try posting on the Edinburgh, Midlothian forum for 1851 census and the Dumfermline, Fife forum for the 1861 census, maybe you will have more luck there.
Liz