Mop Tops

[Celeste]

Help! We have 3 mop tops.. they were all planted at the same time.. (2015) each summer the end one dies back seriously... plenty of water being put on them all.. why?? And what can I do to fix it?
He’s on the left

 

[Logan]

Perhaps it's just not as good as the other 2 or the ground is different in that area.

 

[Sheal]

Do you feed them at all? Even though they are in the ground and well watered, there's a possibility that the water is washing all nutrients out of the soil, leaving very little for them to feed on.