I do not think you need to worry yet.
The usual incubation length is 21 days. Even among bantams, many of them hatch at 21 days (yes we hear about the ones that hatch early, but not all of them do.) Sometimes they even hatch a bit late (especially if the incubator temperature was a tiny bit low. Being just a little cool, over the whole three weeks, can slow them down by a day or two.)
Also, how are you counting the days?
The day you put them in is day 0 (they have been in for no days)
The next day is day 1 (they have been in the incubator for one whole day)
Day 7 is the same day of the week that you put them in (they have been in the incubator 7 whole days, which is one whole week.)
Day 21 is the same day of the week as you put them in (they have been in the incubator for 21 days which is three weeks.)
If you count differently, then the expected hatch day will not be 21.