NimbostratusHi, usually this can be caused by a few simple issues. The most popular is failing to define the failover and config sync IPs for the device. Sounds like that is complete. The next popular issue is the time on the devices is greater than one minute apart. Ensure they are using NTP and less than one minute in difference. Also, be sure you are adding the peer to trust domain by using the management IP address.
tmsh list sys management-ip
sys management-ip 172.24.112.20/24 { }
One last trick that sometimes works if everything is correct above. Add device_1 to trust domain from device_2, wait a few minutes, then add device_2 from device_1.
/var/log/ltm can be helpful. This solution covers everything related to device group troubleshooting if you want to become well versed on the topic.
http://support.f5.com/kb/en-us/solutions/public/13000/900/sol13946.html?sr=33650106