Hi Rob,
At the top of the 6) integrate panel, you’ll see
Light to stack 1 of 2
probaby, set the slider below this all the way to the right, to stack all frames. I will set this to 100% by default.
APP gives a quality estimate of your frames, and that quality is used to determine which frames are the best in stacking. Using the slider, you can easily eliminate some bad frames if you are working with lots of frames.