Anyway can you try the xine backend. It may be useful to check ~/.xsession-errors also to see if there is a relevant error message in there.
]]>I just stumbled about the same thing - ie. playing some file (all mp3s, all with the same rights, no special chars in names/any of the id3 fields) sometimes does not allow to change current position (position marker moves, but little icon to the left of the bar [part of the track position widget] is grayed out).
Funny thing is, if i doubleclick some other files in no specified order, sometimes the same tracks are available to seek, sometimes not - I'm not sure if it in any way relates to phonon backend (but i use GStreamer anyway).
any help would be appreciated.
regards
bat@Phobos $ juk --version
Qt: 4.7.2
KDE Development Platform: 4.6.2 (4.6.2)
JuK: 3.6