Most outside nests aren't successful, too many things can go wrong, for example if the Hen gets off the nest too long and the eggs go cold, they die, or if the Hen is bothered too much she will abandon the nest (and again, the eggs die), or if the eggs get rained on excessively before the Hen goes broody on them they can become infected with bacteria getting in thru the pores of the shell and not develop or die shortly after starting to develop. Then you have predator issues that also come into play. Some of us have great luck with pur Hens brooding their own clutches, but most of don't, which is why we incubate.
A few more males in your flock might get the job done a little better for ya if you have low fertility rates, especially if your flock is mostly free range, because they tend to pair off or form trios. Some of your Hens may not be getting bred, but supposedly a male can cover 4-5 Hens easily (I think this is the magic number for a breeding pen set up tho)... I have a very productive breeding flock of 11 Hens and 5 males, that only free ranges in the afternoons and had 100% fertility from their eggs last season... (they are not laying this season yet tho).
Your male to Hen ration may not be the problem with incubation tho... Have you cracked open some eggs to check for fertility before you incubate a batch of eggs? How old is your flock? What kind of incubator did you use? Other things to consider are the egg collection, storage and handling prior to incubation, the age of the eggs, making sure the incubation temp and humidity is set correctly and the incubator being set up in a room that stays stable with no drafts or temp fluctuations, and also the lock down humidity % can all be major contributing factors too when you have a bad hatch rate.