If, when you first put the bra on, you have fabulous cleavage then walk about and a few minutes later it isn't a fabulous cleavage as it seems your boobs have wandered around, is it just the cup size that is too big?
Iv lost a little weight and gutted my bras seem a little big (I hoped it was just that they are old so stretched) I have just tried on a new bra and the above happens. It's nice and tight around the ribs but after a few minutes the nice cleavage has gone as my boobs seemed to have 'moved'?!?! I will go and get re measured when I can get to the shops of course... just wondered.