This thread has photos of what to look for with fertile eggs. Sometimes when you crack them, that spot is on the bottom. I very gently use a spoon to turn the egg over to see it.
Fertile Egg Photos
https://www.backyardchickens.com/forum/viewtopic.php?id=16008&p=6
How do you know they are not fertile? Are you candling them? If so, how long has the broody been incubating them and what are you doing to candle? And how did you store them before giving them to the broody?
A lot of the time a mature hen will not allow an immature rooster to breed her. The age of your hens is a very good question. Also pullet eggs are sometimes a little harder to hatch than eggs from more mature hens. I've had some good hatches with pullet eggs but you can sometimes have problems with them.
What breeds do you have? Certain thick-feathered breeds occasionally have trouble hitting the target. This does not mean each and every one every time without fail will have problems, but breeders often trim the feathers around the vent on Orpingtons and Cochins for example.
It's also possible he has not yet got his technique down real well. He may not be hitting the target.
Nutrition can play a part in fertility and hatchability, but that is seldom a big problem for us. As long as they are eating a balanced diet they should be fine from the nutrition aspect. If they can forage for some of their feed, they are doing even better.
Other than possibly trimming some vent feathers on the rooster and/or hens, I don't know anything you can specifically do to improve the odds of fertile eggs other than give him time to mature.
