Problem with connecting to incoming calls

Problem with connecting to incoming calls

I've set up a Group to answer enquiries from a widget on our home web page. Zoho Chat has been great but a couple of staff have reported difficulty recently with connecting to incoming calls. This is what happens:
 
"The computer starts ringing and the notification box appears on the bottom right of the screen as it should - when I click on 'accept' the notification box disappears, but no chatbox appears. After this happened the first time… I got a colleague to test it by trying to contact me from a nearby PC. The same thing happened, but once I logged out and logged in again it worked fine. My colleague got the countdown message until it timed out and told her that no-one was available right now, which was frustrating as I was sat right there trying to accept the call. Once I logged out and logged in again it worked fine.”
 
Any idea how we can rectify this? Thanks.