Hi there
For some reason today I cannot split clips. I type S and nothing happens. I right click on the clip and I see that Split is greyed out. Any ideas why this might be? I cannot split any clips in the project. They're not clip locked at all. What have I done to cause this to happen? Help. I'm stuck.
 

Are these MIDI clips or audio clips?

They are audio clips.

Turns out I'd created a vocalsync clip and tried splitting it which obviously failed. But why that should then stop me from splitting any clips in the project I have no idea. I have deleted the vocalsync clip and all is now well again. Bizarre.

zoffmeister
 I have deleted the vocalsync clip and all is now well again. Bizarre.

Not bizarre at all.
Next time simply right click on the Vocal Synch clip and choose "Bounce to clips".
All this does is embed the Vocal Synch data into the clip which will then allow you to treat it like any other ordinary clip.

I'm using VocalSync for the first time now.
Sounds just like something I'm likely to try.
Good to know.

All Region FX are like this. Melodyne, Drum replacer, Vocal synch. 
It drove me crazy (a few years back) when ARA integration was added to sonar and I couldn't figure out why I couldn't edit these clips after creating a Region FX. Its because the clip goes in to a locked state and must be bounced to clips in order to unlock it.