Compare the downloaded video to where it ends and to the same point on the Tivo. Any glitch will cause the download to stop. You can pause it after the problematic point and resume download, but not all programs will resume downloads.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!