The most likely reason that the space wasn't feed up is because you have an open file handle. The space will only be freed up when nothing is holding the file open. So restart the sppapp the file belonged to and the space will probably become available.
Run lsof to see open files. There is a post on removing files which haven't freed their space
You can also write nothing into the file before deleting it. eg > /path/to/file or echo "" >/path/to/file note the single > which writes from the start of the file instead of appending (>>) to the end.