Banana Pi R4 won't boot from eMMC

I'm following the "Getting Started" documentation and everything goes great until I get to the "boot from eMMC" step. When I change my jumpers to eMMC and power on, the whole system hangs - just two red lights on the front panel and that's it.

Any idea what the problem could be? I'm a bit of a noob, not sure where to go from here :confused: