Shrinking a SQL Server 2008 Standard database to move it into SQL Server 2014 Express
Normally the thing holding my data files back from being shrunk is that a CHECKPOINT is needed.  I have had to do several iterations of:

CHECKPOINT;
GO
DBCC SHRINKFILE(...)
GO


Before the file would finally shrink.

If the data files still doesn't shrink, try the troublshooting steps here to confirm that you really have the free space that you think you have.  Here's the query from the docs link:

SELECT name ,size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS int)/128.0 AS AvailableSpaceInMB
FROM sys.database_files;