You have to keep on people sometimes. You can't be pushy, but you can follow up with them, generally people don't mind. There was one piece of artwork that I wanted to trace and I could only find one collector with the game. It took about a month and a half of follow up to finally get photos. He gets a lot of requests, and didn't want to waste his time. Once he saw I was serious, he emails photos to me all the time of other projects. (That, and once he saw the quality work I do.)
So, long story, keep on them. Or offer some sort of trade. Make a small offer to buy photos to pay for their time, or barter a skill or something if you are a computer savvy person.