Besides, 1,2kb seems very much the size of a single preset, so I think I might be rights.
What happens is that the librarian waits for the first sysex message, then starts to store what it recieves... When that message is done, it stops... But the P6 simply start to send the next preset in the bank, but now the librarian has stopped looking for more data.
I bet there is a way to let your librarian take multiple messages in one go... Check its manual.