Suggestions for debugging techniques for complex SMPS IC?

[dsg123456789]

Hello, I'm working on a board design using a MAX77960, which is a buck-boost converter for DC/DC power conversion and lithium battery charging.

I've got the MAX77960 working on a board where I only stenciled & reflowed its circuit, leaving the rest unpopulated. I'm able to tie the MAX77960's power output to the correct place on a board that doesn't have the MAX77960 populated, and with that I'm able to power the 2nd board from the 1st via the MAX77960 (by tying the power outputs and grounds together).

I also have a board that is fully populated. If I inject the 3.3V power rail via a bodge (disconnecting the board's 3.3V supply), I'm able to have my microcontroller communicate with the onboard MAX77960. Its status registers say it's healthy, but the switcher isn't switching, and the output voltage is very low (0.9V) even though it's configured to try to maintain 9V on the output. I can see that the MAX77960's internal LDOs are working--they're both reading 1.8V. And like I mentioned, I'm able to reliably communicate with it and program it via I2C.

The fully populated board has 5 additional connection to the MAX77960:
- 2 control signals from the uC to the MAX77960 that have 10kΩ pulldowns, and which I have disabled/left floating in the uC, so I don't think these have any effect
- 1 open-drain signal from the MAX77960 to the uC with a 10kΩ pullup to a 3.3V rail that is downstream of the MAX77960. This 3.3V rail powers the uC.
- An I2C bus, which has 5kΩ pullups to the aforementioned 3.3V rail

I'm not sure how to approach debugging this:
- should I treat it like a switching power supply that's failing to bootstrap, and look into replacing/retesting passives that I may have damaged during soldering? I've spent a fair bit of time with my hot air station and fine tipped iron ensuring that MAX77960 is attached fully. Are there other approaches one can take to diagnose a non-working SMPS when you know that the circuit/PCB has had successful assemblies?
- Should I treat it like a power design problem, where my I2C pullups are causing weird current paths and breaking stuff? How do I approach thinking that through?
- Is this a partial failure mode that happens with SMPS controllers when cooked too much during assembly, where I should be trying solder a new one?
- I don't think the problem lies in the digital logic space, since the comms seem to be working, but maybe someone has an idea for what else I should be looking into?

Thank you for your ideas in how to approach debugging this! If it would be helpful, I can post a system diagram, simplified schematic, or the full schematic (but the board has 263 components organized in 15 modules, which isn't conducive to specific suggestions).