Multi Track Filter not working

Total newbie question here. I think that I had gotten this to work before, but I just can’t get things lined up this time.

So, basically, I’m trying to get a green screen down. After all of my edits, I’ve got 5 video tracks. The topmost track (v5) is the one with the green screen. The next track down is the one with the video that I want to overlay onto the green.

I select the v5 track, and click “+” on the filter and add the “Chroma Key: Simple” filter. Nothing happens. (In this tutorial, it happens automatically: https://www.youtube.com/watch?v=bpYBj9dByqU .)

I looked at this topic (Green screen does not work when both the back and foreground are landscape or portrait) and made sure that my composites are properly set. (That is, the top track (v5) only has 1 diamond with “Disable Compositing” as the pop up text, and the other track (v4) has 4 diamonds with “Composite” as the pop up text.)

So, for giggles, I tried to do a different filter. This time, I tried “Size and Position” on the top track (v5). It shrunk OK, but behind it (where I would have assumed tracks underneath would be) was just black. I double checked and both of the tracks have open eyes with “Hide” as the hover text. Just to see what happened, I clicked the eye icon on the top level and the underneath track (v4) was displayed.

Now, the top track was a computer generated mp4. Maybe the aspect ratios aren’t the same as the footage that I have with my camera?

I’m using Shotcut version 18.03.06, if that helps.

Thanks in advance!