Usually yellow leaves mean overwatering or underwatering or lack of fertilizer. If you have been watering often let them dry out completely.

Then again it may be the location you chose. Planting under any tree is not a good idea. A perennial may survive but an annual doesn't have the roots to compete with tree roots for moisture and nutrients

Perhaps the dreaded downy mildew. Look for white on the underside of the leaves. If you find it, the plants have to be trashed and the soil cannot be used again for impatiens because the spores can overwinter.

Here is a link that might be useful: Downy Mildew