Trash not freeing up space

I've read a bunch of threads about this, both on this site and others, and I haven't found any solution, so here it is.

My MacBook Pro has been running out of space a lot recently. So I've been trying to get rid of old stuff I don't need and more stuff to iCloud to make space. I just copied my iMovie Library file (iMovie Library.imovielibrary) onto an external disk and then deleted it. It was almost 80 gigs and I only had about 5 gigs free at the time. After deleting it I still only had 5 gigs free. Nothing changed. If I right click Macintosh HD and choose "Manage Storage" the space used by Documents (where it was located) has gone down 80 gigs, but if I right click Macintosh HD and click "get info" or go to About This Mac and choose storage the available storage has not changed. How do I get the storage back? I can't..delete it again to try and fix it because it's gone.

Thanks,

Seth