onDrop move from one slot to another

Hi, I have several instances of a UI object as a 'slot' created from a prefab, which each have a script attached that implements IDropHandler. I also load a draggable game object into that slot. However after starting up, it only registers the onDrop for the slot that it started in, and not for any other slots that I drag it to. The draggable object stops blocking raycast onBeginDrag and starts again onEndDrag.

Any help appreciated! I feel like it might be a layer or group setting somehow?

Thanks