I want to start from the beginning to see what i did wrong.....i made mmf one day and placed it in the fridge like i always do, the next day i baked the cakes. the day after that i started decorating the cakes. i made a 3 layer, 2 tiered cake. before i buttercreamed the bigger cake, i placed about 6 or 7 bubble tea straws in the middle of it. i buttercreamed them, set them aside on my table and started coloring the fondant for the 1st tier. it took about an 45 mins to an hour to do that. then i started covering the cake. i used a smoother to make sure it looked good. i cut it flush with the cake board. then i moved on to the next cake, colored the fondant then covered it. i didn't stack them right away, i did that hours later because i wanted to decorate them first.
after they were decorated i stacked them. they looked great. about an hour later i saw some sagging with the top and bottom tier (the fondant). by the next day, the day of the party, they had sagged so much that i could've cut off about another inch of fondant. what did i do wrong? does the cake need to settle after i put the fondant on? should i make it one day and decorate it the next day? please tell me what i'm doing wrong. thank you in advance for your help.

I don't us MMF, but it sounds like it was too soft.

PS you put supports in *after* the fondant.

thank you, i'll try that next time.

I fill and crumb coat my cakes while frozen...then let them sit out until they settle (typically 24 hours). Then they go to the fridge to firm up. After completely cold and firm, I fondant (with Michele Foster's Delicious Fondant recipe). Then I put supports in. Finally, I stack, stake down the middle and back to the fridge it goes. Haven't had a problem with this method.

Yes, supports should go it after the fondant is on. For one thing, they leave a hole in the fondant so you know where they are during cutting. I'm sure it tasted good anyway....

You don't have a photo do you? Could be a lot of different factors and it would be a lot easier if there was a pic

in this picture, it looks decent, you still can see some sagging. the next day it sagged so much more. i probably shouldn't have stacked it the night before.

It could be that you're rolling your fondant too thick. I found that Michele Foster's fondant can be rolled thinner, thus it's not weighing down the cake.

after watching fabulous cakes on tv, i think what i'm doing wrong is that i don't refrigerate my cake after i bc it. and i noticed them putting another coat on just before they cover it with fondant. is that what i'm doing wrong? i always put one coat of buttercream on, should it be 2, and refrigerate the first layer?
i've been trying to add a pic, but for some reason i can't!

i finally attached a photo, but i put it in the galleries. its the team umizoomi cake.

I think it's either that you didn't let your cakes settle before covering them with fondant or that your supports may have been too short since you put them in before covering with fondant. I know it doesn't seem like the difference in height is that much, but it could be enough that the top cake sinks into the bottom instead of resting on the supports.