Shrink database is not shrinking as much as expected on a 1.5 TB database.
There will always be some free space in the database, stored within pages that are too full for other data. Shrinking databases is an incredibly intensive operation, and will take a lot of time as it moves almost every record to somewhere new. A terabyte is a lot of data. Interestingly, you may have more luck creating space by applying compression such as the Hyperbac stuff from Red Gate. That can help remove the free space and compress the rest, but potentially doing fewer writes (and certainly not having to do lots of log activity) in the process. Your mileage may vary though - I don't want to try to speak authoritatively on how Hyperbac compression of your data will work without knowing much about your data. Hope this helps though...