Update 10.10.5 -> El Capitan, installer doesn't continue after reboot

I'm using the App Store to download the installer for El Capitan. It's downloaded fine, runs fine, then asks to reboot my Mac as expected. After reboot, my Mac just continues as if it was a normal boot, no installer continues. If I run installer again, the whole process repeats.

Gigabyte Z87X-OC Motherboard (Socket 1150, Z87 Express, DDR3, SATA 600, ATX, Haswell, Supports 4th Generation IntelCore Processors, GIGABYTE UEFI Dual BIOS)

Asus GeForce GTX 760 DirectCU II OC, GTX760-DC2OC-2GD5...

Update 10.10.5 -> El Capitan, installer doesn't continue after reboot