I have an odd problem. I have 5 servers running in our office: AD/DC/Mail server, web server, NAS, database server, firewall. All are Linux Slackware 14.2 except the dbserver which is Windows 7. We had a recent several hour power outage in our building. Upon power restoration, all servers powered back on except 2: Mail server with Gigabyte Z97X-UD5H motherboard, and NAS with Gigabyte AB350-gAMING 3-cf motherboard. The odd bit was that "AC Back" was set to "Power ON" in both BIOSes, but they didn't power on. One of the other servers, webserver, also has a Gigabyte MB, B250M-DS3H-CF. This server did come back up after the power restoration.

Other servers/motherboards that resumed OK were ECS 61H2-CM, and ASUSTeK P8Z68-V LX.

Does anyone have any ideas as to why these two servers did not restart after power was restored? I need to find a solution as it is important that all servers come back online after a power event.

Do you have any event logs in BIOS?
Generally, if the machine is not on UPS, I leave "AC back" to power off incase if power fluctuates off/on...

I didn't know BIOSes had event logs! These are production machines, but next time I reboot one (which will be soon) I'll check that.
Since these are production servers, they have to come up ASAP when power is restored. However, I do have a two minute delay at power on before Linux boots. This is for that same reason: power fluctuations. If the fluctuations are more then 2 minutes apart, well, it will go down again. Hopefully the apcupsd software is able to handle a graceful shutdown.

I've found out that both of these machines are on the same APC UPS, model: Smart-UPS 1500. One server (MAIL) is directly connected via the USB data port and the other (NAS) is connected to MAIL via network. All computers on other UPSes restarted just fine. I went into the BIOS on these two computers and clicked on "AC Back" - 'Power On' again, even though that's what they were already set to. I then yanked the wall plug on the UPS and waited for both to shutdown. Several minutes after shutdown, I plugged the UPS back in and ... both computers came back up! So, unknown as to why they didn't do so with the real power outage.

I will conduct another such experiment in the next few days and examine various logs and see what's up.

LATER ...

I shutdown one of the servers and when I turned on the power it came right up. I checked the BIOS (AMI version F8) and saw no option to view an event log. The shutdown test was not the same as a power outage and battery depletion, so not an exact test, but my yanking the plug to the UPS test yesterday did work. I can't explain why these servers did not power up after the REAL outage was restored. I'll leave this open until another event happens along.