How many ft-lbs of torque does your impact wrench put out? Bigger wrench? I only say this because mine seems to work fine but my son has one that does not seem to work on the hard jobs.
I agree with Tom's comment regarding the hammer. And will offer the following:
With full tension on the puller, take your lump hammer 16oz or better and hit the end of the puller in line with the axis of the crankshaft. One good hard shot should be all it takes. But again, the blow needs to be directly on axis with the crankshaft.
I am attaching a photo of our puller set up on an Intruder engine.