No no no!
Try and avoid thinking in terms of "at fault". The primary goal is to get the work done. Mostly it never descends into blame storming and fingerpointing - so try and avoid behaving like it's got there before it does.
You don't need to "keep repeatedly asking", but you do need to ensure you've communicated clearly. If you asked for clarification and didn't get a response, follow up at least once, and make sure you follow up when you decide you need to shelve the task until you get a response (possibly in that first follow up).
"Hey $boss, I haven't heard back about the question I asked about $task on $date. I can't move forward without a response, so I'll put this task aside until that's resolved. If it's still urgent or critical, let me know, and let me know if I should continue chasing up that question with you or with someone else. Thanks, $communicative-co-worker."
Your last sentence is key - communicate. Both ways. With the prime intention of getting things done, and clear language explaining when things are stalled at your end and what needs to happen to un-stall them. If you're doing all that - you'll make your co workers and bosses very happy, and you'll have all the paperwork you need if things _do_ end up in a blame storming session later on. But don't optimise for the blamestorming, optimise for getting things done and clear communication...