I've noticed that some (many?) suggestions on TIVO don't run to their end. This is currently happening with the BBC Proms that appear on the suggestions list. It also happens with eg old Wycliffe episodes.
It all depends what is happening. I can think of two possibilities:
Suggestions are the lowest priority, so will be cancelled if a planned recording is due to take place.
I don't think they are padded in the same way that the planned recordings are, so if the channel is running late or early, then you may find you'll lose the beginning or end of the Suggestion.