The following are some of the possibles:

The soil is compacted making it hard for roots to make any headway; or it could use some old compost mixed through to hold moisture and provide food.

A spell of very high temperatures.

Roots all bunched up after being in a container???

Either - being drowned in a clay-bottomed hole or being droughted because the soil is too sandy to hold water for long. If the plants were bought in pots - then the original root ball may be drying out too fast.

A persistant prevailing and searing wind.

If you have reason to believe that it's a soil/location problem - draw a big bucket of water and dig up one of the plants. Put it in the bucket while you re-dig the planting hole (a $50 hole for a $5 plant is the rule.) If you see lots of bubbles coming up from the plant in the bucket you have a drought problem. Be sure to water regularly and deeply over the season. A bucket of water, a ten minute wait, another bucket of water which would probably be fine once a week until very hot weather comes in.

If you think it may be wind - put up a temporary shelter or move the plants to a more favourable site.

Another possible is fertiliser burn if the new roots have encountered a deposit of chemical fertiliser. Dig up, refresh the soil in the planting hole and replant.

Elderberries can sit very quietly for a year or so before they shoot up and start tapping at the window. Reliable water and protection from hungry creatures are needed.

These are first-guess possibles. Might you be able to say more about where you've planted them and what's been happening weatherwise?