I make whipped cream frequently, but this was the first time I added "real" alcohol, and it fell within 20 minutes. I put both the bowl and beaters in the freezer until cold. Then, I whip heavy whipping cream by hand mixer until stiff peaks form. I mix in sugar on a 1:3 or 1:6 ratio (depending on dessert) and add 1 Tbs flavoring per 1 cup cream. Generally, it stays in the fridge for a few days before falling, but when I added brandy, it fell within twenty minutes. I wasn't expecting it, since I have no trouble with Bailey's or Amaretto. Is this normal, or did I perhaps mess up somewhere else in the prep? I'd like another stab at making brandy snaps, but I'm nervous about fallen cream with all the work that goes into the cookies.
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.
Ah, I just read the answer to this in The Fat Duck Cookbook. Foams collapse when the water drains out of the bubbles. Ingredients like gums that thicken water slow this, and those that thin water, like alcohol speed it up. Adding the alcohol just before serving (with just a bit more whipping) can help. You could also try adding say 1/8 teaspoon of xanthan gum dissolved in the cream to stabilize it a bit.