Could be :
- too dark;
- overwatering;
- spider mites;
- shock from a new home.

The flowers will only last a week or two at best, but new buds should be growing all the time. Remove the dying ones to stop your window sill looking a mess.

New Guinea Impatiens indoors need as much light as you can manage. In a west-facing window you might need to open the blinds for more light.

Hybrid Impatiens don't like to be too wet, although they do like a good supply of water. Too much water will cause a wilt of the new leaves that cannot be cured by watering.

Spider mites are a pest on indoor Impatiens. They thrive in warm conditions and low humidity. They will cause a pale mottling on the newest leaves, followed by bud and leaf drop. Eventually you may notice a fine webbing between the newest leaves.

Do you notice any leaf problems? Wilting, discolouration, or deformities?