Just a thought, but in all the times I've written flocking apps, I've always used append mode instead of create mode. Perhaps I do this because somewhere I've got bitten by create mode removing the file to create it empty, and that of course would have another locker still holding the old (unlinked) file locked, even though I can now lock the new one.

Try changing your lockfile open to an append, and rerun your tests. I apologize if that doesn't fix it, but it is suspicious.