salix trees

Difficult to say without more info.

Are they in the same area of the garden? Getting the same light etc? Is one exposed?

I've had mixed results with these; I started with a potted standard, but it only lasted two years.

My current one (not a standard) is about two years old and in a border with clay soil, that gets sun for a few hours most afternoons - it seems to be doing okay.

As with any Willow, I've found they are very thirsty. If the poorly one looks dry you could always try adding water retaining granules to the compost.

I have no advice to add I'm afraid ,but am interested to follow this post as I bought a standard willow Salix today ,so fingers crossed somebody can help :)

Willow do need copious amounts of water and probably would do better in the ground.
We have two weeping willows planted on the same day, one near the pond is so big you can have a party under the canopy.....the other by comparison is not near the pond, its growing but not a patch on its big brother.....

Could be something as simple as why, in human life, does one sibling have poor health and the other one is healthy and survives. That's nature.