Your bra is stretched beyond the final clasps
Bras stretch over time. Because of this, it's smart to buy a new bra that fits on the outermost set of hooks, so you can maintain a firm fit when it inevitably loses some tension over time, according to Adore Me. If you're already on the tightest set of clasps, but the bra band is all loose and wavy around your chest, then consider discontinuing that particular bra.
[Photo via @adoreme]