Cannot curve edges when combining shapes on Adobe Illustrator iPad

I appoligize in advance for my rudimentary explanation. I am trying to create a pattern for a card holder. I have combined a rectangle and an elipse in order to create a divet after hitting "minus front". Unfortunately, I cannot curve the edges of the divet. I can do it for the original corners of the rectangle, but not the ones created from combining the shapes. Any advice is appreciated.