Ever run into this before? I have, but it's been a long while and I don't remember what I did to resolve it.
 
I have an instance of Kontakt with 4 instruments, and it cannot be frozen. I click on the Freeze button, the "busy" cursor appears for about 2 seconds, and then nothing. It's as if SONAR found some reason the freeze could not be performed, and aborted.
 
I don't see anything unusual about these 9 tracks (5 MIDI, 4 audio, none are Instrument Tracks). Doesn't matter what freeze options are enabled or disabled. 
 
 

Yes, I have had where the freeze button for a track is greyed out (along with the freeze button in the synth rack to the corresponding track). When all the other tracks are fine.
I can't remember how I fixed it, but you might try Unselecting everything and see if that helps.

Ive had freeze problems if i deleted outputs or had archived outputs... any chance this might apply?

Keni, you da man! 
 
I had previously been using a Kontakt instrument for bass but switched over to Trillian and deleted the instrument from Kontakt. However, the old audio track was still there and was archived. Once I deleted it, the tracks could then be frozen. I'd been scratching my head over this one for hours!

Excellent! Glad it turned out easy to fix,

Can you explain this or provide a repro?

I had the opposite issue today...I opened an older project which has several frozen syths but there were no longer any options to unfreeze them.

Good to know.

Spark2 by Arturia has the same issue - won't freeze.

I cannot speak the this specific issue, however, I have had issues with unfreezing or freezing a synth.  Even though a synth was in focus the option to freeze or unfreeze were greyed out.  In all cases, I simply changed the focus by selecting a different synth and then returning the focus back to the synth I wanted to freeze or unfreeze.  Once I did that the option was no longer greyed out and I was able to complete the operation.  It's as if Sonar thinks I originally did not have the correct synth selected.  Moving to a different synth and then back again always works for me.