Teacher, are these sentences OK?
We are returning the agreement you have signed as the person who witnessed your signature forgot to sign it. Can you ask him to sign the witness section and then return it to us promptly.
We are returning the agreement because it is missing the signature of the witness. Please obtain the missing signature and resubmit as soon as possible.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.