I would look at the number of top layers, and outline/perimeter shells layers you have. On your layer settings depending on what software you are using you can define these. alot of times they are low.
Top Solid Layers
Bottom Solid Layers
I think personally the safe min is 5/5/3 but I tend to go as high as 10/10/5 if I want it solid. A little more time and plastic depending on the model really don’t mean that much.