Do not waste your time on the Decrypting ---------
Open up the Windows Event Logs --- in the Application Logs ----
Both Symantec Endpoijnt Encryption and Whole Disk Encryption will write a log for the failure reason ------
Typically BAD sectors or Blocks on disk --- too many and it will pause and stop encrypting.
Best best scrap the device ---- by doing a full reformat of the drvie and re-image --------
If it the drive is in use have the user move all critical files to a share ------ and re-image.