Hi, welcome to the forum. Glad you joined. And relax, you are probably OK.
Are you counting the days correctly? That's is a real common mistake. You don't say "one" when you put them in the incubator. It takes 24 hours for them to have finished one day of incubation. An easy way to check that is that the day of the week you put them in is the day they should hatch. If you put then in on a Friday, the 21 days are up on a Friday.
Many chicks don't hatch at exactly 21 days anyway. There are several potential reasons for that. Heredity, humidity, how and how long they were stored before incubation started, and just difference in individual eggs, even eggs laid by the same hen, can make a difference. One big factor is average incubation temperature. It's not a spot temperature but an average over the entire incubation. If your incubator is a bit warm they can hatch early, as much as two full days. If the average incubating temperature is a bit low they can be late.
Nature does not require the temperature to be exact for them to hatch, she wasn't that cruel. Of course you want to be as close as you reasonably can but she gave us a nice window that works. A half degree C is within that window.
Once this is over I suggest you calibrate your incubator and adjust incubating temperature before you start the next batch. The instructions that came with it should tell you how. If you need help, maybe telling us the make and model can help us help you. You do not want to do that while you re incubating, you stabilize it before.
In the meantime don't touch that dial. You can't do anything to help, you might make it worse. And, I'll say it again, if you are within a half degree you should be in good shape temperature wise.