I tried applying quantization when I played these top parts in via my MIDI keyboard, but that was not successful. So I went into the Key Editor and applied and locked quantizing there, but I am still not hearing the parts lock to the notation nor play together in rhythm.
What am I doing wrong (other than playing the part in in step-time in the future)? 
margaretsWaltz241210.dorico (946.8 KB)
Reset Playback Overrides seems to fix it on my end. I wonder why reapplying quantization didn’t work?
When you quantise in Dorico, you’re quantising the notation, not the underlying MIDI performance. It’s completely the opposite operation of what you might do in a sequencer.
Yes, but shouldn’t locking to the notation in the Key Editor override the “flaws” of the recorded MIDI performance?
I’m not sure what you mean by “locking to the notation”, but if you mean switching the Key Editor between displaying the notated durations or the sounding durations, no, that doesn’t change anything about playback – it controls what happens when you edit notes in the piano roll.
When you have it set to notated durations, editing the durations in the piano roll changes the real durations of the notes, as they appear in the score; when you have it set to played durations, editing the durations in the piano roll doesn’t change the appearance of the music, but changes the MIDI performance.
I thought there was a way to tell Dorico to ignore the played version and stick to the notated rhythm. Perhaps I am using the wrong command (or cannot find the right one), but the parts are not together and are not in rhythm (due to my inaccurate playing to the click).
Hi @Derrek, it seems that you want to eliminate the inaccuracies of the recording (instead of just temporarily disable the played durations). If so, the suggestion by @DanielMuzMurray is the way to go:
Select All > Play (menu) > Reset Playback Overrides
Yes, thank you. That is the command I could not find/recognize.