What check is it using.
This information is important for troubleshooting, as inside the check is defined what data should be fetched from the device.
Inside the Juniper list of checks is only the “juniper_trpz_power” and “juniper_fru” i think relevant for PSUs.
Can you please provide the output of an SNMPwalk on the OID “.1.3.6.1.4.1.14525.4.8.1.1.13.1.2.1” or “.1.3.6.1.4.1.2636.3.1.15.1”
I think the second one is the only working on your device.