I wanted to see if my data could be described better with a non-linear relationship. So I calculated a new Y' variable, based on the coefficients I got from the regression model. Just to make sure I didn't make any mistakes, I rerun the regression, with only the new Y' as the predictor. The results were the same. Then, I used the curve fit option in SPSS to look for a better relationship

However, when I try to do the same thing with an interaction variable, this doesn't seem to work. The regression loses a lot of R^2.

Why is this and what can I do to make it work?

Thanks