Is it ok to write back when no response has been received from a postdoc application?
During academic job applications, I did not hear back at all from at least two places (in the UK), so I would not assume you will hear back. On the other hand, there was also the situation where I had not heard back from one of my top choices so I contacted them and they told me that someone was supposed to have contacted me two weeks earlier but somehow didn't. It wasn't good news, but I had been stalling on agreeing to another position so ultimately it was helpful to just ask, politely obviously. I really don't think it would hurt your situation, and as others have mentioned, may even help it.