There are different brands of shortening, some are thicker and heavier than others. Depending on what consistency you want, choose accordingly.
Crisco tends to be heavy and thick,
Walmart's store brand is light and fluffy.
There are also differences in butter, some stay thick and some are softer when whipped.
All these differences will effect the final outcome of your frosting.
I often make the frosting a day ahead of time along with the cupcakes, but pipe the frosting the day I'm serving the cupcakes.