After years of experimenting the only tool I now use for digging down in difficult ground is a Post Hole Bar - Chisel End 7.7kg 1.75M. It will cut through anything and give you deep narrow holes if that's what you want.
Made out of tempered steel, it is also the best leverage bar for removing heavy stones or roots.
Sharpen with angle grinder as required.
I only wish I had got one 30 years earlier!