They say ideal is 99.5, I always set mine at 100. Sometimes it goes up a little (like up to 102), but always keep it around 100, not below.
Maybe they're getting cold?
Are you using your own eggs or ordering eggs?
If you order, try to get from someone not too far (like not the other side of the country), and rest your eggs (at room temp) for a good 12 hours before you put them in.
102 will kill your chicks