Why can't I get the available free space after shrinking?
In shrink action, you need to choose:

"Reorganize pages before releasing unused space"


And choose desidered size.

Actually you don't have released space because you have still used pages ad the end of your file.

Edit:
You can do it via query, with instructions like this:

DBCC SHRINKFILE (N'MATRIX24X7' , 449000)