ADF4169 MUXout abnormal

I have two adf4169 boards, which use SPI (3.3V) of MCU  convert to 1.8V logic to control their registers. The performance of the two boards are different.

Board 1:  VCO output can be locked  by adjusting the current value of loop filter and Icp current(0.3mA).By writing the muxout flag of the R0 register, we can get the correct r-divider,but no without n-divider (at this time, I can phase lock).  When I asked muxout to output dvdd, muxout did not have the expected 1.8V, and VCO lost its lock. At this point, if I write 0x6 to the R0 register, the system can resume locking, but muxout still has no output.

Board 2: the system is unable to lock the VCO, and the muxout control is abnormal. Sometimes the r-divider can be captured, sometimes the n-divider has a signal (but it can not locked VCO), but the dvdd has not been.

The timing of SPI has been checked and seems to  correct. The two boards have the same hardware.

What is the next step, could you give me some advice?

thanks

Lan

BG8AAK