I tried again on the other side of the panel (my aquarium is a peninsula), and it worked flawlessly. To rule out human error, I also retested it on the previous panel where it didn’t work yesterday—but unfortunately, it still didn’t work.I use mine on 3/4 with no issue. I have even used it on 1 inch. Maybe yours is defective? Make sure it's aligned properly.
It seems that the panel with the overflow might be slightly thicker, just enough to prevent the pump from functioning properly. It would be interesting to test with an Ecotech MP60, but I already have enough flow and wouldn’t want to spend that much on another pump.
Other than the glass thickness, I’m not sure what else could be causing the issue.