I have found that when an address has an apostrophe in the name (mostly occurs with Suburb rather than street names) the API return
"Unable to calculate route."
If I manuaaly remove the Apostrophe (and don't leave a space) it works fine.
An example is;
From: 99 Wakefield Street,Adelaide,SA,Australia
To: 4 Majors Road,O'Halloran Hill,SA,Australia (doesn't work)
4 Majors Road,OHalloran Hill,SA,Australia (works)
4 Majors Road,O Halloran Hill,SA,Australia (doesn't work)