BeagleBone Black not booting

[Tzuriel]

I've followed this wiki page (which is excellent, btw), to create a microSD with Gentoo for a rev. C beagle bone black. Had no problems and it appears to have been built and installed perfectly fine on the card. https://wiki.gentoo.org/wiki/BeagleBone_Black

My problem is that it doesn't boot from the micro sd card, only from eMMC. Here's what I've been doing:
1. bbb is powered off.
2. insert card.
3. hold down boot button nearest card.
4. while boot button is down, tap power button.
5. at this point I'd keep the boot button down till lights blink, but they never do.
6. release boot button.

I should add that I made the following changes to the wiki page which is starting to get outdated.

- Grabbed a 4.4 kernel.
- Used a 64GB micro SD card.
- 'sfdisk' has since been updated. The 'sfdisk' line in the script mksd-am355x.sh no longer accepts -D, -H, -C, and -S, so changed that line to " | sfdisk $DRIVE". Both partitions build fine.
- I used the very latest u-boot, v2016.05.

Thoughts?