Stacking png files from Unistellar telescope

The Unistellar telescopes produce png files that are already integrated with a dark and with multiple lights from the scope's internal stacking.  But several owners then stack multiple integrations to try to increase the signal to noise ratio. I have tried multiple times to do this with APP but the result looks much worse than either of the individual integrations. Note: I am stacking, not creating a mosaic. Has anyone tried this, and what is your workflow? Thanks!