Flipbook won't add new sprites

Hello Community,

recently in Unreal when making a new flipbook, I can’t add another sprite to it that wasn’t previously selected in the content browser and/or was created just before I made the flipbook. Quite strangely it can still add older sprites.
Maybe it has to do with my sprites or images( I’m using PNG-images) or maybe I’m doing something wrong with my flipbooks.

Does anybody have some advice on the matter?

Many thanks in advance.