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Many parts of the country are experiencing high heat, which can stress new plantings. If this is the case where you live, I would increase watering to several times a week, for 15 minutes or more each watering session.

Prune your weak plants back and go easy on watering. You might find the following article helpful: https://www.gardeningknowhow.com/ornamental/shrubs/photinia/growing-red-tip-photinia.htm

According to the growing zone for your state, https://www.gardeningknowhow.com/planting-zones/maryland-planting-zones.htm, you are in Zone 6 or 7. Red-tip photinias are hardy in zones 7-9. If you happen to be in Zone 7, growing these shrubs should be fine (a zone lower or higher is also acceptable in many regions).

Mulch is used around the plants as opposed to mixing it into the soil. It holds in the moisture, breaks down and improves soil. Adding compost (does come in bags) to improve soil is always a good idea. The mulch may be making the soil too course thus not allowing the plants enough water.

Red Robin, or Red Tip Photinia, gets its name from the bright red young leaves which appear each spring on the tips of every twig and stem. Red Robin is a fast-growing, vigorous evergreen shrub, with attractive deep, glossy leaves. Photinia glabra rubens is the finer leafed variety. It is a little smaller than the other hybrids and easier to trim.

Feed it with a slow-release, balanced fertilizer (8-8-8) and one that is aimed at acid-loving plants. Fertilize once in spring and once in early fall.