Emails get lost, most often in Junk Mail boxes.

When I don't hear back from someone within a reasonable time frame, I follow up with a short email asking if my previous email about this and that was received. If I do not get anything back either within a day or two, I call with exactly the same question: "I sent you an email about <subject> on <date> but have not heard back, so just want to make sure it found its way to your inbox". More often than you would think the person I called would either find my emails in their junk folder, do not find them at all, apologize for overlooking them, or apologize for not replying sooner. (And sometimes they would say they replied and I would find their replies in my junk folder.)