Does the database consists of multiple files and/or multiple filegroups? When you want to shrink databasefiles using the GUI it will default show only the first datafile of the PRIMARY filegroup. Please execute the query below to show the size and usage of all files within the current database. Post the results if you need additional assistance.

SELECT

[name]

, DB_ID() as [DatabaseID]

, [type_desc] as [Type]

, /128.0 as [SizeMB]

, fileproperty([name],'SpaceUsed')/128.0 as [SizeUsedMB]

, (/128.0) - (fileproperty([name],'SpaceUsed')/128.0) as [FreeMB]

, [physical_name] as [FilesystemPath]

FROM sys.database_files