This sounds dumb, but you have to manually record these playoff games. Including manually setting the channel and start and stop times. The Tribune guide data has been pretty bad this year. On my stand alone Tivo, it usually just says "NHL playoffs Teams TBA" but sometimes it has some other show instead.
Go here to see which channels might have it.
That's correct, the guide data is a mess. Blame NBC for that.
With NBC holding the rights to the games, you can't record them on the RSN. NHL Center Ice does not apply for the playoffs. And game search won't work, because the "correct" channel often has the game listed as NHL playoff with no teams specified, so game search can't find it. Nothing to do with the HR20, and nothing to do with DirecTV.
And FYI, my HR20-700 is the BEST DVR I have owned, apart from the HR24. My HR21/22 DVRs have all gone back to DirecTV.