This has been answered a few times,
Basically, since the channel merge, some channels are coming from XM's DC studios, some are from Sirius in NY. Basically all of the songs being broadcasted from XM's studios are not compatible with the seek on Sirius radios, it doesn't have the right info embedded in the songs. Here is a link to a topic that talks about this at DRC.
Which channels are coming from DC? - Digital Radio Central
there is a list of what stations are coming from where, so you can get an idea. Hopefully as time passes they will merge the playlists and make them all seek available.