Evidently your senior does not follow a task checklist when doing something as important as a depromotion/promotion of a DC. (we all make mistakes)
Let's assume you set the new Domain Controllers TCP/IP DNS settings to look at ITSELF. You did do this, didn't you? Go check it in the NIC settings.
(I'll also assume since the promotion, that you've rebooted the machine once or twice.)
First run Netdiag at the command line. Copy and paste the results to notepad and see if anything needs correction.
If all is well,
Then run DCdiag at the command line. Copy and paste..........
You know the drill.
Edit: Here's a specific FSMO command: DCdiag /test:Knowsofroleholders /v
Also find out more FSMO here: FSMO Role finding