We need more information about the value of "new".
One thing to watch out for is that the default 'linear' method does not extrapolate: only points interior to your "a" can be interpolated and points outside what "a" defines are going to be NaN. You could try specifying a different method but it is not clear that it will help.
If this is the problem then you could switch to the relatively new griddedinterpolant. But watch out for your MATLAB version:
The behavior of griddedInterpolant has changed. All interpolation methods now support extrapolation by default. Set F.ExtrapolationMethod to 'none' to preserve the pre-R2013a behavior when F.Method is 'linear', 'cubic' or 'nearest'. Before R2013a, evaluation returned NaN values at query points outside the domain when F.Method was set to 'linear', 'cubic' or 'nearest'.
