"Prefers well-drained soil" is on more plant tags than any other requirement, which is probably why it gets read as a suggestion. It is not. Roots need oxygen as much as they need water, and in saturated ground they suffocate and rot — and a rotted root system kills a plant that is being watered faithfully, which is why the diagnosis so often goes the wrong way.
Ground here works against you in two ways. Heavy clay holds water and gives it up slowly. And in places there is caliche — a hardpan layer of cemented calcium carbonate, sometimes a few inches down, sometimes a couple of feet — that water simply will not pass through.
Which produces the mistake we see most often: the bathtub. Somebody digs a hole in heavy ground, backfills with good compost-rich soil, and plants into it. Now there is a pocket of loose, absorbent material sitting inside a bowl of material that does not drain. Water runs in, collects, and has nowhere to go. The plant is standing in a container with no hole in the bottom, and the better the backfill, the worse it is.
Test it before you plant anything. Dig a hole about a foot deep, fill it with water, let it drain, fill it again. If the second filling has not drained within four hours you have a drainage problem, and no plant choice will fix it.
The fix is to plant up rather than down — a raised bed or a graded berm, so the root zone sits above the ground that does not drain. That is a real solution. Amending a hole is not, however good the amendment.
One thing worth knowing before choosing a plant list for a dry site. Most of what is sold as drought-tolerant also insists on sharp drainage, because the plants evolved on gravel and decomposed rock rather than on heavy valley clay. Put one in slow ground on a lawn irrigation schedule and it rots — which is a xeric planting failing from too much water, the opposite of what anybody expects.
