After completing the first 8 practice levels, you need to create a new jam from scratch to get some more hearts to unlock the next set of practice levels. Note that you will need to use the 'Studio Mode' to create your own jams.
To create a new jam, go back to the main screen (with the record player) and press the "Studio" button. Go into the studio door on the right, and you can start making a jam from scratch.
And for reference, here are some materials to help you get better acquainted with Coding Jam: