If the AxiDraw returned to the home corner, then it sounds like the plot finished *normally* in some sense.
My best guess is that one of the following things happened:
Either (1) the software either determined that there was no more data to plot and returned home or (2) the entire plot ran, but the pen wasn’t touching down for part of it. (If you were asleep, then that allows for that second possibility as opposed to quitting in the middle.)
For (1) you should run a plot preview to make sure that everything that you expect to plot renders correctly in the plot preview. If there are parts of the document on a non-printing layer, outside the printing bounds, issues with interpreting the file or planning motion for it, or other similar things, this should catch them.
For (2), it could be that the pen isn’t touching everywhere it needs to in the file (if so, it will seem to “fade” as it more lightly touched the parts near to those missed), that the pen-lift servo motor isn’t working reliably, or that the pen ran out of ink or dried up.
One test that you could do to determine the difference between these (if everything else seems okay) would be to try and run the same plot with the pen-lift servo disconnected on the left side of the machine, so that the pen is down for the entire travel. If there’s a question as to whether the entire path was drawn (but the pen up/down wasn’t working), this could answer that question.
