Motivation could be an issue. What if they do not want to communicate with us? What if we are too slow for them? Dolphins produce burst pulses that have interclick interval of just few milliseconds. We cannot even hear these individual clicks and for us the burst pulse sounds like a creaky door. But they, supposedly, can hear individual clicks. We do not understand or use echolocation, they do, so their "dolphin-socrates" probably would have their own allegory of the cave that is not based on what individuals can see.