Disappearing notes

As I was going over the drum score of a project, I noticed that I had the HH playing at the same time as the ride cymbal. It was on the wrong line, as it was supposed to be pedal HH. In a triplet section, I dragged the notes down one line from HH to HH pedal. and when I looked at the score, it was already set, so they moved to the space near the BD instead of the RC line.

I looked for similar situations in the score and found likewise where there are 16ths in the RC. I selected the notes as before and dragged down, but instead they all disappear instead of moving down one line as before. I don’t know why.