It might be a nitrogen deficiency. Mulch leaches nitrogen out of the soil as it decomposes. Before you put your mulch down this year, try using a nitrogen fixer on the soil (like what you'd use for beans). You'll have to play around with what proportions are best for your soil/mulch combo, but I'd be willing to bet that's your problem.

Adding legume inoculant will not add nitrogen to your soil! The bacteria need to be in symbiosis inside the roots of specific legumes to fix nitrogen - they can't do it on their own - and the nitrogen only becomes available when the legume plants die and their roots decompose.

The idea that decomposing mulch depletes nitrogen is not universally accepted anyway.

Maybe the mulch is keeping the soil too moist. It should not be more than a couple of inches deep, and keeping the mulch from directly touching the plant stem will help with this.

More information on what kind of mulch you use and how you apply it would help people advise you better.

Maybe you are using too much mulch? Summertime mulch can be laid very thin.

Since the impatiens are going to spread, leave a few inches around each plant clear of mulch.

well, I guess that's the last time I'll listen to the nursery man! Thanks for setting me straight, annebert.