Drag-and-Drop is not working.

On Mac this problem is only known to occur in older versions of GarageBand. However there is a simple workaround to this problem:

Simply double-click the item you want to drag and drop. The process would go like this:

Double-click -> Hold down -> Drag and drop

Alternatively updating OSX and GarageBand also solves the issue, since in the latest version of the DAW you only have to click the item once for dragging and dropping it.