Well, the answer is simple - the item that you're trying to update with new status is not locked by your process.
That could happen in many, many ways. For example:
- The item was already updated
- The item ID was overwritten with some different item ID
- The previous item has been marked as exception, and the process received a new item id, but it has not been locked. You cannot update a new item, because it has not been locked at the start of the process.
- The item has been deferred and that action unlocks item.