You must reattempt service until it is effectuated. The witness has no obligation to contact the server unless the witness wants to make it easy and accept service.
Unlike serving a summons, and serving a subpoena must be by personal delivery on the witness. The idea is that, because, the consequence of disobedience could include jail, we can't just allow someone else to tell the witness about substituted service.
Witness isnt obligated to call server and make it easy. The witness must be served personally and the papers cant just be left with someone else at the residence. Must keep trying to serve until successful or you give up.