Would you care to post your recipe? There might be something in there that helps us figure out if lack of fat is the problem.
The first thing I thought of was time spent in the ice cream maker. At the restaurant we make all of our own ice creams and sorbets, and one thing we do to keep them softer and lighter is to mix them longer. This incorporates more air which will stay trapped as it freezes.
I think the other thing that helps us a lot is that all of our ice creams start out with creme anglaise as a base, and there's plenty of fat in that. If you're not familiar it's basically cream, egg yolks, and sugar. It doesn't make for very healthy ice cream.... but hey, if you're eating ice cream then you're indulging already, so you might as well do it right!