Rectangle Frame Tool in A-Parent

You should be able to drag any file from Bridge, Finder or Explorer into the frame even if it is not selected or the loaded curser if you place a file.

The only cause I know is if above such a frame which is covering up filled with a little bit of text. Check your layers on Parents and on Page if you see something.

To select objects on a document page that are resident on a Parent page they must be overridden onto the document page. Ctrl (Cmd) + Shift click on the object to override.

That said, you don't need to select a frame to Place content into it. Just hover the loaded cursor inside the frame and click. It will override automatically.