Many shows do not allow F.F..
There is normally a notice at start of the program. At any rate,; some allow F.F. during the actual program. If you try to F.F. during a commercial the commercial will continue to play. But; the program is actually F.F. the part of actual program after the commercial.
This is what happens if you try to skip commercials on programs not allowing it.
I have had this intermittently happen where pause, FF, and even REWind would not work. If you stopped playback, it would restart from the beginning. It has happened twice over the past year or so. It affected mulitple recordings on multiple channels. It only affected one evenings recordings, luckily.