Did you try different foods? I feed my discus beef heart and brine shrimp on alternating days. I doesn't look like you have a pair from the pics, because you only have that same one on the right side. And if it was a pair, their body would be twitching as the female looks for a smooth area for her eggs. But it could be a number of reasons why they like the right side better. One could be the air stone is scaring them. Try relocating it and using a wand instead of a stone to even out the bubbles. Or you have a lot of water movement on the left side, discus like calm water which is why many people use sponge filters. Or they just plain like that side better than the other, flip flop the DW to see if it makes a difference. And I'm only seeing 10, did you lose two??