A Weigela Bush can be dying due to root rot, which is caused by overwatering or poor soil drainage.
You should water the bush once a week (ideally in the morning) thoroughly for the best results and make sure the soil has good drainage. The soil should be moist one inch below the surface.
The Weigela Bush is also susceptible to a fungal disease called Twig Blight.
Don't wet the bush's leaves while watering it. Spray the plant with a copper fungicide or lime sulfur fungicide and increase the air circulation around the susceptible plants.
Hope this helps.