Smart Pianist music library on iPad stopped playing through piano

My Yamaha P-S500 piano has a specific port for an iPad set up. I use a genaric lighting cable, which worked fine until recently. Now, only the built-in songs in my Smart Pianist app play through the piano. The purchased songs now only play through the iPad speakers. The lights over the keys on the piano still run with the melody, so something is coming through. Just not the sound of music!