Hello, m.gray,
the problem you are describing may indicate that the session key file is missing in your PC.
Normally, AES_NI ransomware creates a session RSA key in the C:\ProgramData folder. The name of the file is like this:
JOHN-PC#9C43A95AC27D3A131D3E8A95F2163088-SPECIAL NEW-201752671610-234.key.aes_ni
Please, take a look if you can find such file there.
If yes, please, send it to decryptors-at-avast-dot-com.
If not, try to search your PC for all *.key.aes_ni files.