When you insert notes in the score editor, the key editor automatically adopts the same length settings as the last note entered. It also adopts the same quantize setting as the score editor. If this last note is a long one, and you then want to add expression, modulation, etc. you first have to change the length and quantize in the key editor to a larger value (= smaller notes), otherwise you can’t draw in gradients. This is a major source of frustration.
Apologies if this has been resolved: I’m still using Cubase 5 because, apart from this one feature, it works perfectly for my work flow. I’ll upgrade when it"s been sorted!