A room temperature crumb coat is key.
I'm not sure what kind of frosting you used but a crusting buttercream with a stiff consistency works best for me.
Add less sugar, also I've heard adding salt helps
Oh I did mine with cream cheese frosting, turned out great!
I use Swiss Meringue buttercream exclusively, and they are perfect for swirls. As the other poster suggested, leave your crumb coat at room temperature. And if you are using a crusting BC, do not let the crumb coat crust, or else you roses will not have anything to adhere to and will eventually fall off. And definitely use even firm even pressure, making sure your rosettes are fully attached to the cake surface. Good luck :)
I crumb coat my 1M rosette swirl cakes first, then attach the swirls to the crumb-coated surface. I do NOT put the swirls directly on the cake since they would fall off.
I've only had one rosette fall off. I quickly washed my hands, lightly rubbed some moisture on the spot where it fell, carefully re-attached the swirl and nobody could tell anything had gone amiss.
I freakin' LOVE these cakes! They are so impressive and everyone thinks you worked like crazy. Here are some I've done with American buttercream:
https://christinascakes.shutterfly.com/pictures/447
https://christinascakes.shutterfly.com/pictures/401
Using Pastry Pride (a non-dairy whipped topping):
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