Dear Dan,
Hitting snags now and then is a normal part of the learning experience. I would recommend you go to the addition section of the Math Card Games book and play some more games to strengthen her basic skills. I love the Corners game, Rows & Columns, Addition War, and the Short and Long Chain Solitaire games. The more she plays these the better her skills become and then she can do the mental addition. Sometimes parents find it helps to take a break from the lessons so their is no pressure and then the natural love of learning will return. Relax, enjoy playing together and the brain will create new synapses.
Nancy

I have heard some people like to use Math Mammoth for worksheets for extra practice. The Blue series works on specific skills.