Does adding brandy to whipped cream make it fall quicker?
Anything that dilutes the cream is going to make it less firm/stable - whipped heavy cream can last for hours, while whipped light cream (such as Reddi-Whip) won't last more than five-ten minutes. I've never had any whipped cream last for days, however, unless it contained a stabilizer such as gelatin.

1 tablespoon brandy per cup of cream does sound a bit much - just for comparison, you'd use maybe a third that much vanilla extract for the same amount of cream. (You were probably using much less Amaretto, and Bailey's contains cream so it doesn't dilute things as much.) If you can't get the flavor you want with a reduced quantity of brandy, you could try making a hard sauce (butter + alcohol) to serve with your dessert, in addition to or instead of plain whipped cream. Or you could try a whipped cream stabilizer - either the stuff you can buy specifically made for the purpose, or unflavored gelatin carefully dissolved in a part of the cream.

Note that for best results, you should add the sugar and flavorings to the cream before starting to whip. It sounds like you already know to keep everything as cold as possible.