Looks like you missed the stud and need drywall anchors.

Are you sure the wall itself is plumb?

Beverly

What do you mean plumb?

Drywall anchors... thank you!

PJ,,you don't have the proper supports for these. you just screwed them into drywall, yes? the drywall anchors or attaching directly to the studs will hold better.

Plumb means level up and down. walls sometimes lean forward or back and are 'out of plumb'.

Perhaps you can show us what the back of the shelves look like and how they fasten to the wall. Some floating shelves have rods that anchor to the wall, then you slide the shelves over those rods. The cheap version of floating shelves just have hangers that hook over screws held by little plastic anchors in the wall. Sometimes, the metal over the "keyhole" on the back of the shelves is so flimsy, it actually bends when any weight is placed on the shelves.

You need some tenacious wall anchors if not attaching to studs, I would shim the backs of the shelves so they lean in to the wall to compensate for gravity.