You said you added a zone to your server, why? Is the server that handles dev.yourdomain.com not the same that handles yourdomain.com? You shouldn't have to create a separate zone file if the same hosts are managing the parents. You don't mention what server, so I'm going to assume bind, but most dns services will support doing something like this:
dev IN A 126.96.36.199
admin.dev IN A 188.8.131.52
These entries both appear in the main zone file. If however you are planning on hosting DNS for the dev.yourdomain.com subdomain somewhere else, then you need something a little more complicated. Setup your zone file as you have, then add references from the parent servers:
dev IN NS otherns.yourdomain.com.
dev IN NS backupns.yourdomain.com.
As for the actual issue of some hosts not resolving, have you run a trace? Is it all the time? Are they querying the same NS hosts? Have you validated the configurations on multiple servers, if you're running multiples?