Changing audio outs - sometimes yes sometimes no

Hi
I have a strange problem that I am not sure I understand.  

There are times when I add a VST instrument that it will not let me change the output audio channel.  Never happens on just an audio channel on its own.  Does happen when there is a VST in a folder track - ie the MIDI track and the Audio.  

I can close Sonar, open it back up and then things work.  It does not appear to be specific to any particular VST, though it does seem to happen quite a bit when I am using Stylus RMX (though not on the Stylus tracks - but on new instrument tracks).   Ditto on Kontakt when I am opening up a SSD Drum thing - no problem with the Kontakt outs, but problems with new synth plugs afterwards.  

I have 2 RME FF800s on a FF800 and an i7.  and a ton of other stuff.  Running 8.5.3 Producer.