I opened an old X1 track with the idea of substituting AD for an SD drum kit.  Created a simple instrument track, copied the midi track to it--works fine.  But when I tried to edit the velocity, or anything else for that matter, in PRV, I couldn't get the cursor to change anything. I opened another Midi track in the PRV window and it operated normally.  The original SD Midi track has the same problem, however.  It's a though this drum track is locked, somehow.  And, no, it's not that little "lock" button in the track pane, if that's what you're thinking.  I even copied the track into a new project, in case it was a corrupted file, but it did the same thing.  Am I missing something? 

Well, just in case anybody else ever runs into this, here's the answer: Bounce To Clips.  Never mind.

Thanks for the update.  I did not chime in because I had absolutely no idea why it was happening or even a hint of solution.  I will keep this in mind if it ever pops up

When in doubt, bounce.
 
That's kind of like the old Windows standard: when in doubt, reboot.
 
[EDIT] I just realized that no one is likely to understand the reference...
In other situations (and as discussed in other threads here) Melodyne has behaved strangely and the workaround seems to be: bounce. If the same thing works for this situation, well... you know...