There are several reasons your transplants die soon after planting.
The hole is too small. The answer is to dig a hole much wider (but not necessarily deeper) than the original root ball of the plant, so the plant can quickly and easily grow roots out into the loosened soil.
Planting too high. When a plant is planted in the ground with the potting soil exposed, it dries out almost immediately, even if the surrounding soil is still moist. The solution is to plant the top of the root ball just slightly below the soil line and place a half inch layer of native soil over the top of the root ball to seal over the potting soil. It needs to be soil; mulch is too coarse to provide the needed air barrier.
Mulch in the planting hole. Gardeners plant in an area that has been mulched with bark or other coarse compost. The mulch is scraped away, and a hole dug for the new plant. Unfortunately, the mulch falls back into the hole during planting, or the gardener inadvertently mixes mulch into the soil while filling in the hole, and now the plant is sitting in a mix of soil and loose mulch. The mulch, being very coarse, dries out at a quick rate, so the plant dries out quickly as well.
Not Firming the Soil. Too much air in the soil makes the fine root hairs of the plant dry out before they can penetrate into the native soil. The answer to this problem is to ‘firm in’ the plant gently by pressing down on the backfill around the plant with hands and some body weight if the soil is fairly damp, or even a carefully placed foot if the soil is dry enough not to compact too much under this degree of pressure. The final step is gently to run a slow stream of water around the plant from a hose, to help reduce any large ‘crumbs’ that did not get taken care of while firming in. ♣