lorax

Check to see if they're rootbound or if the soil is compact. If either is the case, re-pot them.

Equally, they die if they get too hot - over 32 degrees C makes them all droopy. I don't know if that's the case in the window where you're growing them. However, if seems very hot, try moving them to a slightly cooler area. They don't need a whole lot of direct light, and at least here they do much better in the shade.

Naranjilla are grown in the steppes of the cloudforests here - areas that see full sun perhaps once or twice a year at best. This means they're ideally adapted to diffuse light and lots of water in the air. Ideally, if you're growing Naranjilla indoors you should be watering twice a week and misting daily.