Both devices in standby

I've set up a device group with two LTM machines (this is the fourth of these setups I've done so far). I had created a device group, and it seemed to be working, but when I changed a preference setting (HTTP to HTTPS) on both devices and synced the config, it put both devices into standby. I've tried to sync both devices, but they're both still in standby. I even removed the device group and peer list entries, and started over from the beginning, but I can't get either device out of a standby state. Any ideas to what could be causing this?