Need help!!!!

Are you putting supports in, to take the weight of the top tier? If so, maybe you are cutting them a little bit too short?

I use the bubble straw and cake broad

Are you letting your buttercream set up a bit before stacking layers? or maybe it’s not stiff enough to begin with. Beautiful bow!!!

It’s usually a weight distribution issue – is cake covered in fondant? As suggested, your support straws may be too short – in the future, you may want to cut your bubble straws just a tiny bit higher so the top cake will rest on the straws and not directly on top of your bottom cake. Also, could it be that cake is in a room that may be too warm?