When to get involved with employee annoyance

I have a question that I can't seem to find a great answer for. I have found many sites that offer tips for how to resolve conflict between 2 employees, however I struggle to find resources for times when one employee is frustrated at the other, but the other employee is unaware or has no idea.

Here's the situation with 2 of my directs:

Employee A let me know that they feel frustrated that Employee B hasn't got back to them regarding an email Employee A sent several days before. This led to Employee A telling me that Employee B has done this before and it is very frustrating. Employee A deserves to have their inquiries answered in a reasonable amount of time, but Employee B has no idea they are not responding quickly enough. 

How do I as a manager handle this scenario? I don't want to play "parent" and get on Employee B for something they didn't know they were doing that was negatively effecting Employee A. I want to encourage Employee A to let Employee B know how it effects them negatively when they don't respond to their inquiry soon enough. If that conversation either doesn't go well, or things don't improve after that, I would get involved directly.

Should I be getting involved more in this scenario? As it stands, I recommended Employee A to give Employee B the chance to "know" about the issue before I have to get involved. 

Thank you!