Guacamole connected, waiting for response for a while then disconnected

[do hung]

Hi,
I have this problem with Guacamole. After logging in, I got a message saying 
Guacamole connected, waiting for response and after about 15 seconds, it 
disconnects. 
This problem only happens with few workstations(no matter what browser was 
used), not all. I works very well with other workstations that I try so I guess 
I have configured Guacamole correctly. 
Can anybody help me out?
ThanksHung