Measuring XTAL (XI, XO)

[caliper15]

Something totally different which could be a problem (thinking of what you said about oscillating for 2 seconds and then stopping): is the reset line of the mmc maybe floating?

The mmc doesn't have a reset or enable pin. I've checked all the power input pins and they have the correct values (3.3V and 1.8V) and have decoupling capacitors. I have a previous board using this same circuit and I can measure 12Mhz from both XI and XO. The only real difference I can think of is the different board layout.