A few questions...

1.) Does your soil drain, or does it have a tendency to stay wet?

2.) Was the soil it came potted in, peaty? It's possible that you may think you are watering your plant but the soil your plant came in is not holding on to water. Peat moss soils are notorious for this--especially when exposed to hot, sunny conditions.

I would take it out of the ground, loosen up that root ball some so roots are not girdled and replant.

Don't feed until it looks happier if at all.--these plants do great in arid, low fertility soils.

They don't need alot of water. Half strength fertilizer. Poor variety? Mine grew like crazy when I lived down south.