Just as an FYI- I have had eggs hatch on day 25/26 (and I know exactly when she started setting both times), so they can be later than day 21.
THe other thing to note is that soetimes the broody will eat the egg, either because she can "tell" it's a dud, or she just does. I have had some that get a hole in them, that they will then eat and others that they just do for whatever reason. Make sure that she has plenty of protein available to her, that setting, esp. in this heat, is hard work