Changing bearings on track hubs

Following this advice to extract the worn bearings from my one-one track hub:

To remove cartridge bearings you need to knock them out from the inside.

Easiest way is to strip off all the locknuts etc from the axle then use said axle to knock out the bearings. Thread a tracknut onto the end of the axle to protect and use a bit of wood twixt mallet and axle.

I'm upto a 4lb sledge hammer and the axle is still not budging.

What am I likely to be doing wrong?