I've had this problem, too.
Scanning for Qualcomm HS-USB QDLoader 9008 device...
Download Port: Qualcomm HS-USB QDLoader 9008 (COM37)
Connection status: EHCI:HUB:HUB:USB 2.0 High-Speed
USB Driver: [Qualcomm Incorporated] qcusbser, version: 2.1.2.0, date: 8-31-2015
Initializing...
MSM Serial number: 0xd7ac6401
MSM HW ID: 0x000db0e1 (SDM710)
OEM Public Key Hash: 0xcd55540dcb175cc80762cebf08fafd1232c75d13fd06444048ae94bc94a6132d
SBL SW version: 0
Firehose: prog_firehose_ddr.elf
sha1: 8d0a544f8fa5fdc59c4c709a576f031d7fb26785, size: 701.66 KiB
QC_IMAGE_VERSION: BOOT.XF.2.1-00092-SDM710LZB-1
IMAGE_VARIANT: SDM670LA
OEM_IMAGE_VERSION: mz-builder-9
Sending firehose loader... Done.
Handshaking...
RF id: 0x0
Secure boot: on
Lock state: locked
PSN: 872QZ61E8E00C183
Handling remote authentication... INFO: Authenticate failed!, Chance rest : 2
Done.
Sending ping... INFO: Authenticate failed!, Chance rest : 1
failed