This issue can occur when one of the following conditions are met:
Limited number of IP address available or exhaustion
All IP addresses are being currently used so no IP can be assigned to new tunnels until a connection terminated. When an IP address become available, then the IP address will be assigned by user.
All IP addresses are set as preferred IP addresses
This occurs when the total of possible users is greater than IP addresses available. Over time, the user records will be populated with preferred IP address entries. If the system reaches the point where all the available IP addresses are set as preferred IP addresses and the preferred IP address is in use, then an unused IP address will be assigned (which will be another user's preferred IP address).