There are 2 important ways to make sure that your instructor will receive your assignment:
Use appropriate file names.
When submitting documents (whether uploading a document to the dropbox or attaching a file to a discussion post), use only letters and numbers (no symbols such as %, &, ", or ') in file names.
Complete all steps of the process.
When using the dropbox to submit assignments, always make sure you see the File Submission Successful message on screen before leaving the dropbox area. When using the dropbox, you will first locate the all files you wish to submit, then upload them, and finally, submit them.