I'm not using Armbian, but had same issue and google brought me here as I was facing same issue.
As mentioned tkaiser, issue was for me in u-boot script(/boot/boot.scr), where I had: "setenv bootm_boot_mode sec" and I've compiled sunxi-next kernel. When I changed it to: "setenv bootm_boot_mode nonsec" and recompiled it with "mkimage -C none -A arm -T script -d boot.cmd boot.scr" it gave me 4 online cores after reboot.
Hope it will help others facing same issues.