I've found the knockouts in NM boxes hard to fully open. I've had trouble with both Carlon boxes and Madison Smart Boxes. The plastic molded to make the knockout is very resistant to actually being knocked out.
My current approach is to whack the knockout with a hammer's claw. This generally busts the knockout pieces apart, but it doesn't remove them. They often remain so tight together still that forcing the cable in is very difficult and I worry about damaging the cable sheathing. Grabbing the "arms" of the knockout with pliers and pulling and twisting generally just crushes off a little bit, rather than wrestling out the entire chunk that's in the way.
Sheathing damage was a major problem when trying to push old, brittle rag wiring through the knockout: it was stripping the brittle insulation off the wires rather than admitting them into the box. My solution there was to drill through the knockout arms to get them out of the way, but this was a major pain, and the burning plastic smell generated by the rotating drill bit was not encouraging.
What's the "right way" to fully open these molded knockouts?