MATLAB: Plot of confidence interval with fill

confidence intevalplot

Nothing wrong, you just happen to have very narrow confidence bounds.

Use the patch function instead:

figure
patch([x; flipud(x)], [curve1; flipud(curve2)], 'r')

producing:

You may have to use two y-axes, with the new yyaxis or older plotyy functions.

Also see the documentation on Axes Properties (link) if you need to adjust either one to create the scaling you want.