It could be a bad transaction, which aren't always the easiest things to find.
The first thing I'd do is go through the VOID interface in EDC and see if you spot anything irregular. If you're lucky, you might see something that looks totally out of place in one of the data fields.
You could also call your processor and see if they can tell you where it's getting hung up. Some processors/help desk people are more skilled and helpful than others.