Process Servers can read, can't they?

Why is it so difficult to get people to follow simple instructions?  When I send a subpoena to a process server as a rush matter, I include a polite request to fax or e-mail the proof of service back to me.  Inevitably, they do not, put the original in the mail and failed to keep a copy for themselves.  I've worked with hundreds of process servers in dozens of states, and ALL OF THEM do the exact same thing.  What the hell am I doing wrong?  Is there some secret I don't know on how to get process servers to cooperate?