Uploading file problem

I'm very new to PHP, so I hope no one is offended by reading my very basic questions. I have created a web form that saves some data to a database and uploads three files, copies them to a new directory, and renames them. From my work machine and home machine, it works great for me - there are never any errors. When my boss tries it, two of the files work fine, but the other fails at the copy. I have watched him do it, and it's not user error. What could the problem be? I'm stumped! I can include code or links if needed. Thanks, Amanda