Hello Philip and Andi,
I hope you won't mind if I come in on this thread. The Sibelius Mixer in my 14 or so years of using Sibelius with VSL has been one of the biggest problems with getting playback to be reliable. It is temperamental and easily thrown off.
It still seems to be the case that if you load two instances of the same instrument the Mixer will put them on the same channel. This can be a problem in any Vienna library where there is no dedicated second violin(s) - for example, writing a string quartet with Solo Strings before Solo Violin 2 was released. I encountered it again recently working with the Dimension Strings I. I wanted to load the Group 1 / Group 2 presets for a first violin section and second violin section so each had its own staff (so 4 staffs in all). The Mixer put all of them on channel 1.
I also recently tried using this 5-staff string section: Dimension Violins, Chamber Violins, Dimension Violas, Dimension Cellos, Dimension Basses - using the VE Strings soundset - and that wouldn't play back.
In connection with Philip's changing of the channel number, sometimes this has seemed the only option left, when everything else doesn't work. I avoid it now, but in the past sometimes it did solve the problem.
Andi's advice about saving, closing and opening again with the Mixer on Auto setting is new to me - maybe it should go in the next revision of the Optimizing Sibelius pdf (?). I'd like to see more tips added to that to help those of us (and it feels like we're in the minority) who are relying entirely on a Sibelius score for the playback.
The rather lengthy procedure for setting up playback in Sibelius, and the troubles with the Mixer, make it a matter of regret that VSL are not inclined (as discussed in another recent thread) to create a notation-based playback that was more integrated with its own libraries. That would be a more desirable purchase for me than any further instrumental library or software.
All this still with profound gratitude for what VSL has made possible for notation-oriented composers such as myself.