Sometimes docked apps don't want to open your document, even though they may be able to, so you have to coax (okay, force) them to give it a try.

For example, let's say you created a document in WordPerfect for Mac a few years back. if you drag that document to Microsoft Word's icon in the Dock, chances are it won't highlight (which would be the indication it can open that document). If that happens, just hold Command-Option, then drag the document's icon to the Word icon in the Dock, and you can force it to try to open that document. 

From Apple computer, Tip of the week