[heurist]

Really depends on who is responding. Emails that require a lot of effort to respond to are less likely to get a response in general, I've found.

[nottorp]

Prying the answers you need out of your co workers and customers is a skill. I hate it but someone has to do it so it might as well be me.

Repeat the unanswered questions, add helper questions etc. I've discovered no way to avoid it.