Agents gets unregistered every now and then

I have created two agents in TeamCity and got it authorized. Using those I have even integrated my projects and all has worked fine.

But now those agents are getting disconnected/ unregistered every now and then , and eveytime I have to start them manually. Sometimes even after starting the agent they don't start and in Teamcity it shows as unregistered. I am quite confused as to what to do in this matter. How to resolve this issue?? Can anyone help in this regard???

TIA