They do not eat enough or exercise enough when they are broody. Check her to make sure she has not lost too much weight.I did the floating test and 13 out of 16 floated, so that's a good sign I guess
+ I think that as her first time of being a mother she'll sit more than 21 days because of non good experience of moving the eggs slot under her!
The float test is to show wiggle movement in the eggs. It is not the best way to tell if chicken eggs are alive. It is about the only way to tell if emu eggs are alive. If there are chicks still alive in the egg, you should hear them chirp.