Insufficient water will be the problem - new shrubs like this will need quite a lot of water planted away from large trees anyway when they first go in, until they've put out their own water seeking roots (over autumn in this case). Willows are particularly thirsty plants though, and do tend to suck up any available moisture rapidly. When you say its 'very close', I'm not sure what that means, but it should be at least 3 feet away, assuming you actually mean Prunus lusitanica - its spread after 10 years will be 6 feet, and that means either side of the main stem, so if its any less than 3 feet, you'll end up with a lopsided plant. And that's without water deprivation if its any closer.

Thanks, Bamboo. It is prunus lusitanica and it's at least 4 feet away from the main trunk of the willow, but also the same distance underneath its canopy (although that will change slightly when the willow has its annual pruning in September). By the way, I forgot to say that the other shrubs I mentioned are also newly planted - the laurel, for whatever reason, is the only one affected, so I'll give it lots more water and see what happens. Thanks again for helpful advice.