Azure DeleteBlob fails authentication MAC signature

This C++ call fails with the following message (seen in Fiddler)

ret = Service->DeleteBlob(ctnr,bname,false,L"",CloudResponseInfo);

AuthenticationFailedServer failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature.
RequestId:b7427700-501e-0025-786f-df52d0000000
Time:2018-04-29T04:07:44.3475782ZThe MAC signature found in the HTTP request 'eclC9KZqc6MGAOOq+CuBaUejrBpyp1tvnePncCfm5EA=' is not the same as any computed signature. Server used following string to sign: 'DELETE

Service is a TAzureBlobService*
It used to work, then it worked for some storage accounts but not others, now I can't get it to work anywhere.

any ideas?