Dragging and Dropping Images from the file tree

[bedrock1]

Most of the time when dragging image files from the tree onto the page, everything goes well. Sometimes however when attempting to perform the drag, the cursor becomes the dreaded "cannot do" symbol of a circle with a slash inside it. When this occurs, I must go through all the steps of the Insert menu to get the image onto the page.
Does anyone know what would cause this to happen and how to avoid it. It seems to occur for no apparent reason and goes away just as mysteriously as it came.