Ask Mr. Smarty Plants

The Native Plant Database identifies three grasses that go by the name wiregrass; Aristida beyrichiana (Beyrich threeawn), Aristida stricta (Pineland threeawn) and Nassella tenuissima (Mexican feathergrass). The first two occur in the pine woodlands of the southeast United States and are not native to Texas, so Mr. Smarty Plants suspects you are talking about the third. This is a very widely used plant in central Texas landscapes.

It's always hard to guess what the problem is with a plant when you can't see it (and often when you can see it), but let's start with the basics. Here is what the Native Plant Database has to say:

"Growing Conditions

Water Use: Low 
Light Requirement: Sun , Part Shade 
Soil Moisture: Moist , Dry 
Soil Description: Well-drained, acid or calcareous sands, loams, or clays. 
Conditions Comments: Requires good drainage and cant take excessive moisture. Should not be watered heavily more than once a week. Goes dormant during drought and in winter. May rot under heavy mulch. Grows well in containers."

Usually, the problems with this plant result from too much water or poor drainage that can be exacerbated by heavy mulch. It doesn't sound like you are overwatering at one inch per week, but you might check to make sure your soil is drying out between waterings.

You will also note that the plant goes dormant during drought. During this past summer in central Texas lots of these plants around town fell over or otherwise went dormant.

Another possible explanation is that some plants are just less vigorous than others and you may have some that are simply predisposed to being small.

My advice right now is to keep up your watering schedule, make sure they are not too heavily mulched and see if the cooler weather brings some improvement.