Constant current souce using LT8645S

I've designed a constant current source based upon the LT8645S.  See attached. It works well when the load is around 1-2Ω.  But when I increase the load to 3-4 ohms, the current drops significantly.

The input voltage is 42V, but the output voltage is only reaching 18-24V, thus the current fall off.

The output of the AD8030 feedback opamp looks to be trying to regulate Vref, but I just don't see what the issue is.

Any help would be greatly appreciated.