I submitted my file to the Digital Dropbox, but my professor didn't receive it. What's wrong?

From BbWiki

You may have used the Add File button instead of the Send File button. The Add File button only stores the file in the your dropbox, whereas the Send File button puts it in the instructor's dropbox.

The way the you can verify is by seeing if the file receipt says, "Submitted by" (indicates sent to instructor), or "Posted on" (indicates simply added to your dropbox). If it says, "Posted on" you may click the Send File button and select the posted file from the drop down list to submit it to the instructor.