Nice, I have monoprice plugs but those Sewell ones looks great, better than ones I'm using.
If the plugs did not fit certain receivers I can bet it is the low end receivers that still use the pinch style speaker connectors, and not the round ones.
As the shape is the same, I can see this being a noob mistake.
AVR: Marantz 7008 Phono: Pro-Ject Debut III Speakers: BW CMC2 + 2xCM9 + Energy 2xCB-10 (rear)
TV: Panasonic 65ZT60, Samsung 60F5300