What are you hoping to appoximate? The blue data, or the black points? If you want to appoximate the black data, then why have you even plotted the blue curve on top? Perhaps the blue curve is what the smoothing spline produces?
I might guess the black data is what you want to fit from the shape drawn in yellow. But since you seem to have a large amount of data at the peak, I fail to see why it is you expect to see that yellow curve at all.
And of course, we don't actually have the data, but only a picture of it. So it is difficult to explain much about what is happening, or should happen. If you do decide to attach some data, please do so attached to a comment, or by editting your question.
