First thing I would look at is Windows able to see all the memory.
If Windows cannot see the 64GB, neither will SQL server.
Not all versions can see 64GB, and Numa architecture actual memory configuration can be a factor.
I had a 2 proc machine, we took one proc out for licensing, and needed to fill certain slots that the one proc could see.
You could override this in the BIOS, but not as efficient.
Then as Gail mentioned, SQL will not use the full allocation unless it needs to.
So you would have to check for signs of memory pressure.