When an Unleashed AP comes up, it will first check if there is any exiting Master already working in the network. If yes, it will join the existing network and if not then it will become a Master.
During this process it will be in "Building Unleashed Network".
Since the AP is coming up as Master, meaning that the AP is unable to communicate with the current Master. There are some of the reasons: 1. Unable to reach the Master. 2. Different firmwares on the new Master and current AP. 3. High latency. 4. Different Country code.
If you still face any issues, you can open a support case so that we can help you.