Glasgow was built mainly upon the earnings and overseas interests of merchants and importers, who were hard hit when America became independent.
They lost not only the capital they had invested in their southern plantations, but also the income derived from their monopoly over the importation of (mainly) cotton and tobacco.
That was when the rot set in with an economic downturn for Scotland, but Glasgow in particular, and this is shown in the age and style of the majority of the buildings still extant.
Glasgow stopped.
The shipyards and engineering industries survived until the 1940's; but after that...pfffft.
How do I know this?
I read about it in a museum last time we were here.
:)