Have a look at these plugs

It takes a good ignition to fire rich or lean mixtures. Following what works is key. You may not have the detail you think, O2 sensor location reading mixed cylinders at one collector .... ? WBO2 is cheap, but may not be accurate to 0.2.

I think you have fuel/air distribution problem among cylinders, and weak ignition. But saying that is a big guess.... I am reading plugs from poor picture with unknown prior use, then reading between the lines on the miss. Take what ever I say as a grain of salt.