hard pic with the sun shining in from the back ...

when did you plant them ... if last year.. did you cut them back in fall or spring???

its not a watering issue.. and they look rather healthy .. and i doubt a disease or bug issue ...

your one problem ... if you have one ... is that you dont have another shorter plant in front of it.. to hide what is probably normal growth ...

btw.. and what i cant see.. what is the short growth that seems to be in the middle of the clump ... if thats new growth.. then i suspect its last years plant.. and should have been cut back in late winter.. or early spring. .. so that all that 'old wood' would have been removed ...

if im guessing wrong.. so be it ...

water when your plants need it.. insert finger.. or use a trowel.. hand shovel or a tablespoon.. and dig down a couple inches.. and find out about moisture at root level.. then water accordingly ...

its really the only way to do it.. because we cant know your weather.. soil.. drainage.. etc.. in your garden ...

ken

There are many different kinds of salvia, some fully herbaceous perennials and others that are really more like woody based shrubs (botanical term is 'subshrub'). Having a bare base if one of the woodier species is not uncommon and has nothing to do with care practices or diseases or pests. Often these can be cut back in early spring just as they start to push new growth to keep the plants denser.