They still get full sun, and we don’t use lawn fertilizer. The one that never blooms could be too deep – no buds develop at all. It’s worth a try to dig it up.
But the overall problem is probably that the foliage is getting whacked off due to mildew and blight in humid July. I didn’t know that could affect bloom – the plants themselves come back and look healthy each spring. Fighting fungus should be my focus, I think, to keep the foliage longer.