I used to make up batches of giant bubbles for my students on the last day of school. I used dishsoap and glycerin. Somehow, the cornstarch use in your recipe puzzles me, as I'm familiar with using it for a thickening agent, and for bubbles, you definitely want a light mixture. My advice would be to leave out the cornstarch and see what happens. Also, you might check other bubble recipes online, compare and go from that.