Some units, if you bypass the regen too many times, it will require dealer intervention to enter regen. This is because the ECM will only allow the engine to run at de rated power, limp mode.
Regen could have been bypassed, or if this unit uses an injector in the DPF to burn off soot, that injector could be bad, or the fuel supply to the injector disrupted. I’d properly warm up the unit, then run it up to PTO rpm and try to start a regen to see if it will complete. If not, then I’d be calling a dealer for service.
DPF equipped tractors really need to spend most of their time at 1800rpm and above to keep the DPF clean. When it’s clogged, the tractor has to run at 2500+ rpm usually to get hot enough to regen and burn off soot