One of the filesystem was 90% full.
I moved one of the giant files over to a different filesystem and compressed it. but when I run df -k on the original filesystem, it still shows 90% utilization, am I missing something?
Any help appreciated

move or copy?

did you delete the original?

There is a process still reading/writing file. What you has casue the original file to be unlink()ed which means that it is no longer visible from the filesystem, however the process which already has the file open can continue to use it. The space on the filesystem will not be released until the process "lets go" of the file ( ie when you restart the process )