for example, call this person and have a small chat with him/her, or even better, walk over to his/her desk and say hi to him/her - build rapport if possible. Stage 2: If the issue still persists, then Step 1: document all the facts and issues (emails, messages, screenshots etc.,). Step 2: talk to other people and see whether other people are experiencing the same issue with that guy/gal. If yes, even better. Step 3: Bring up the issue to your direct manager and explain the situation, issue and (negative) impact/consequence on your or work. Provide all the proofs to your manager.