I cannot for the life of me get sharp corners on my fondant, and when they get somewhat sharp it's only for a few minutes then they seem to round out and sag! Is it because of the consistency of my buttercream? Although it seems to do it no matter how I alter it. Any tips? Thank you in advance for you time!

 It may be sagging because it's too thick and heavy or you have a lot of buttercream underneath.  If you don't already, try putting it in the fridge so the fondant can set. 

I usually apply the fondant and smooth it, then put it in the fridge for a bit to firm everything up then go back and really work on my edges when I can be a little more rough then back in the fridge.

When you say edges, do you mean the top edge? The real secret to a smooth fondant finish is the following:

a perfect frosted cake that is completely level

a cold cake(after it has been frosted) let it sit in fridge for at least an hour

cold envirorment at least 67degrees

roll fondant to 1/4" thickness

use a fondant smoother 

hope these tips help! It can be so frustrating when the fondant doesn't cooperate  

good luck! 

Marianne

white flower cake shoppe

Unless you can keep it cool, the buttercream is going to bulge and sag under the fondant once it comes back to room temp.  Have you thought about using ganache under your fondant?  It was a game changer for me. 

One other thought, I have heard that those who use SMBC don't seem to have any problems with sagging under their fondant.

Nancy

Thank you guys! I am going to work on all of that! I usually freeze them for a day before I frost then again before I cover them. Maybe just the fridge will work better. I've seen some post about SMBT but haven't been able to try it yet, I definitely will! Silly question, but do you put the ganache over a crumb coat of BC? Or just ganache? I would love to give it a try. Again, thank you so much!