It might be waiting for the coolant to reach a certain temp before starting the regen process. Being that its winter, probably not getting hot enough. Try working the tractor hard, high revs and working that engine.
How many hours on your tractor?
I'm not sure about bypassing the DPF, and I don't think thats the issue. If it were plugged up, you should be getting a dash error. Could be the injector is blocked, but again, throwing parts at it in hopes of fixing it, is not ideal. Without these stupid computers, the DPF system is actually pretty simple.