Pumpkins vary in water content. Those with less water tend to be more difficult to process and will often turn out dry and gritty. To combat this issue, go ahead and add water in small amounts. Add just enough to get the pumpkin pureed smoothly and be extra certain to drain as much liquid out of the puree as possible before using. Another thing that will help is pureeing in smaller batches. Food processors are easily overwhelmed by volume and smaller batches are easier for them to puree effectively and efficiently.