What constitutes due diligence to secure the presence of a witness?

has exercised reasonable diligence but has been unable to procure his or her attendance by the court's process." (Evid. Code, 240, subd. (a)(5).) " 'What constitutes due diligence to secure the presence of a witness depends upon the facts of the individual case. [Citation.] The term is incapable of a mechanical definition. It has been said that the word "diligence" connotes persevering application, untiring efforts in good earnest, efforts of a substantial character. [Citation.] The totality of efforts of the proponent to achieve presence of the witness must be considered by the court. Prior decisions have taken into consideration not only the character of the proponent's affirmative efforts but such matters as whether he reasonably believed prior to trial that the witness would appear willingly and therefore did not subpoena him when he was available [citation], whether the search was timely begun, and whether the witness would have been produced if reasonable diligence had been exercised [citation].' [Citation.]" (People v. Sanders (1995) 11 Cal.4th 475, 523.)