As most calls to GrabzIt are done asynchronously they are therefore not necessarily processed at the same time. This is why GrabzIt returns a unique id for each screenshot when you use the Save method so that the correct screenshot can be associated with the correct action.
Therefore ensure that the unique id is recorded of each screenshot or alternatively pass a custom id to the URLToImage, URLToAnimation, URLToPDF or URLToTable methods so that it can be processed properly when it is returned to your handler.