The process of hatching eggs is very simple, but at the same time very specific. Most likely you have incorrect temperatures. Only less likely is that you have incorrect humidity. We will all assume you knew that you needed to rotate the eggs. If you did not That is your problem. I would also assume you had fertile eggs to begin with.
I would also assume that you have not made your first hatching attempt to be shipped eggs.
Read up follow the advice and odds are nearly 100 % that you will have good to fair hatches.
Throw some eggs in a warmish spot wat 21 days and see what happens and your odds are nhearly 0% that you will have any success at all.
The answers to your question lie here on BYC and many places on the web but you must follow the recipe so to speak. Not a lot of room for improv in the hatching world
You could also have bacteria buildup inside your incubator. Miss Prissy wrote a great thread about bleaching out your incubators between hatches. I just disassembled four of mine and bleached them per her instructions this week and have loaded them up again. I won't know for three weeks how they are doing.
If your eggs are fertile, then proper humidity and temperatures should result in chicks.
The biggest/quickest improvement you can make is to buy a digital temperature/humidity gadget from Wal~Mart, should cost you less than $6. Make sure your incubator is germ/bacteria free and your eggs are fertile and you should have chicks pipping 20 days after placing eggs in incubator.
There has to be a problem, or you would be hatching! If you can give all the details, someone might spot the one little thing holding you up. It's not criticism, it's wanting to help. I would love for you to have