I know this is an old post, but I found it when I had the same issue.

I inherited a database that was 150GB and had 148GB free space (the import routines where written by someone with no experience and had bloated the database massivley).

I couldn't get the database to shrink, left it running for 6 hours and it wasn't doing anything.

In the end I suspected it was because none of the tables had a clustered index on them, I added one to each table and then the shrink took less than a minute.

Worth checking if you google this problem and get this page as the first result as I did