You might be entering the commands too slow, or too fast. It can be tricky sometimes. Also, be sure that you're viewing the Now Playing List when you enter the S0RT commands from your remote.
Are you able to get 30-sec skip to work (enter select-play-select-3-0-select while viewing a recording)?
Sony SAT-T60 120GB (v3.5b)
HR20-700, HR20-100, HR21-200 HD DVR
HR10-250 HD DVR 400GB (v6.4a)
H21-200 HD Receiver
R10 DVR 160GB (v6.4a)