I hope I have uploaded two files - one titled autostartPolicypng and the other is startpolicy.png
These are screencaptures from the web-based ASMI - Advanced System Management Interface. I do not have an HMC, so I use the web interface. HMC would have a different way to change these.
Approach ASMI as the system firmware IP address as
https://IP_addressOrHostname and login.
Now you should see - on the left - an area titled "Power/Restart Control".
The panel entitled "Power On/Off System" has a selection entitled "System firmware start policy". This is (mainly) for a normal startup.
The choice you want to look at is in the left menu as: "Auto Power Restart". Make sure this is enabled.
HOWEVER, if you system is managed by an HMC all partitions get started, more or less in parallel - so the ones that are dependent on a VIOS may not start as expected. When you do not have an HMC - the VIOS - as IVM (integrated Virtualization Manager) has that role and will start client partitions AFTER it has started itself - and so the system returns as much as possible (depending mainly on the integrity/availability of external storage).
Short advice: get on the ASMI web interface and make you adjustments. Or use the HMC to do the same.
Hope this helps!
Michael