ADRF6601 LDO not powering up

I have the ADRF6601 on a PCB which plugs into a control card. I have two different configurations for plugging the cards together. The control card writes the SPI commands to setup the ADRF6601. In one configuration, the ADRF6601 comes up as expected and I get the downconversion that I expected. In the other configuration, I do not get the downconversion I expect. The 3p3_LDO and VCO_LDO are at 0 volts.

What would cause the LDO regulators to be off? Is there another way besides through a SPI command to turn them off? I am wondering if my second configuration has an issue with the SPI bus and the ADRF6601 is not being programmed properly.

Thanks,

Mark

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