The most common reason is that an Agent does not reach one of the minimal requirements (free hard disk space , CPU availability, free memory, etc.) specified in the Coordinator settings. To see if this is the case, open the Coordinator application, and check the following columns in the "View->Columns" menu:

Physical memory
Virtual Memory
Avail CPU
Disk Space

If one of the values for these columns is colored in red, this is the problem. If none are colored red, try starting a build and then write to support@xoreax.com specifying what the 'Status' column says for each of the Agents that aren't assigned.

The other most common reason is a connectivity problem between the Initiator machine and the Helper machine.
If this is the case, you should see the following warning message in the Build Monitor | Messages tab: "Failed to connect to NNN.NNN.NNN.NN (port 31105)"

In order to verify if this connectivity problem is related to IncrediBuild or happens due to other reason, please do the following:
1. Install Telnet or Putty client on the Initiator machine.
2. Using Telnet or Putty, try connecting to port 31105 on the problematic Helper machine.
3. In case you are unable to connect to this port, please consult your IT since the problem is not related to IncrediBuild. If you are able to connect, please contact IncrediBuild support for further assistance.