I'd begin with the softly softly approach, as BandR has suggested. If there's no reply I'd give it a week or two then try the same thing again.
If there were still no response it's just possible they've changed their email address and not updated it on gc.com so they might not have received your messages. I'd then look at their recent cache logs to try to identify other cachers in the area that they may know, who might be prepared to give them a gentle nudge.
If that fails I don't think there's really much more you can do.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)