I set up a simple Bind server on my VPS and it is working properly.
The problem occurs with my local windows machines, which are connected to internet through the home router.
I created an A-record named 'dev' and it is invisible from my local network for some reason, though people from other locations can resolve dev.mydomain.com. Ironically, dev.mydomain.com cannot be resolved for myself only.
If I add another A-record, say, 'gamma' then it becomes visible from my local windows machines instantly. So this is just for that particular 'dev' name.
The only difference is that I had dev.mydomain.com server on another IP but that was a month ago; all nameservers have been changed since then. I tried to reboot my router and flushed dns cache on windows machines: no result.
Thank you in advance.