Variants on this question come up a lot: "I sent an email to a professor about X and they didn't reply, now what?"
My untested pet theory is that it's related to imposter syndrome somehow. People in graduate school, or entering graduate school, or otherwise just trying to enter academia, never feel like they belong. You're clearly not good enough to be there, so you had better try not to make waves lest someone notice and rectify the situation. So when a professor fails at something basic, like replying to an email, the automatic assumption is that it's probably a reflection on how little you belong, even in their inbox.
Don't get me wrong, I and every PhD student I've talked to have felt the same way, and I imagine that doesn't change at the post doc level either.
But the general answer given is "what you asked is reasonable and they should have replied. Don't assume the worst, they're probably just busy. Yes, you should absolutely follow up again after allowing enough time to go by from the first attempt."
From what you've said, this advice exactly fits your situation too. Following up with a phone call might be even better - you're not accusing them of anything, or upset they didn't reply. You're just helping them out with fitting you into their busy schedule.
The bottom line for me is that if they are actually sending you an implicit "no" by not replying, it is they who should feel sheepish when you follow up, not you. We all face rejections at various points in our academic careers, and there's no excuse for avoiding giving timely, professional rejections to others when necessary.