I personally found it easier to quilt Heart Flow on a smaller scale on my Chain of Beads Quilt. It could be the space I was trying to fill with this design which was too narrow for the big heart shapes and multiple echoes.
If you want to quilt Heart Flow really big, try quilting it All-Over style so it covers the quilt top completely with this design. If you want more texture for your quilts, or to fit it into narrow spaces, then you'll need to quilt it on a smaller scale.
And here's the quick version of Heart
Flow: