What else is in the tank with her?
Also when shes ready to drop the fry,She will try & hide from the others & start rapid breathing, This is when she needs to go into the box.
Ive had tons of fry over the years & have found that the fry numbers increase after each pregnancy & if you have any plants or covered areas the fry will hide. This worried me at first, But they will find food & hide very well plus they are faster than the adults.
If you dont catch her in time dont worry, There will be more chances next month
She will Thank You for leaving her in the tank honestly