I have worked with my old advisor for 1+ year to produce my senior thesis. I'd say that our relationship was good when I left to do my masters (and he did write me a reference the first time around). I'd say he was satisfied with my studies if not terribly impressed.
I was fairly confident that he would write a letter this time around for my PhD application (1 year after our departure) but now I've sent him two emails without getting a reply. I feel that there is really no reason he would decline other than he is too busy or getting too many letter requests--this may be why he just didn't reply to my email without an outright refusal.
I cannot "ambush" him in person since my visa has expired. I do have contact info to one of his phd students and his phone number. Should I call him or contact his PhD student? Or keep sending more emails? What should I say in the emails? I really need his reference.