Cross-posting can greatly reduce your chances of getting responses.

Volunteer researchers, those who can help you, check your posting history prior to spending their time giving you a hand. If I see someone posting the same, or near same query, all over the place , that signals to me they're on a fishing expedition.

Posting on multiple boards might get you multiple responses, but it's incredibly annoying to find someone already "found" and posted the same data on another of your posts, it's counter-productive and a time waster. Keeping the conversation on one thread helps everyone who is taking the time to help you. Volunteer A find one bit of data, which Volunteer B uses to go further. Volunteer C comes along to join in the competition, and that's what you want.