The end plant, which is doing really well is under the shade of an overhanging tree. Could the problem be too much sun for the others?

I wonder about 'plenty of water'. Rosemary needs good drainage and can manage with little water. How about digging up the worst affected plant and examining the roots? Sun will not be a problem for this Mediterranean native.

I would be concerned about a water issue as well. Does that planter drain properly? Do the plants just receive natural rainfall or are they irrigated? All rosemary needs to thrive is a fast draining, lean soil, as much sunlight as you can manage and as little water as is necessary to keep the plants alive.