When you finish the bow, have you tried running stitch through the center? This is how I've done them, I use either a pin or a needle with thread (which is a much better choice). This is what works for me, I know we all have different tricks, but you can give this one a try (if you haven't yet).
Since the bows are so small it's a little hard to get the hang of them, but if you keep trying I know you will get it. I couldn't do them at first, the center would always look weird when I finished, but I practiced and now they come out pretty cute (if I do say so myself