I have applied for standard visa for my parents. Completed and submitted both of the applications. Now I want to book the biometric appointment together. How do I make sure they get the appointment together?

I was able to book it for my father on 23rd September but when I am trying to book for my mothers application I can’t see the option to select the date. By default a date is getting selected which is 21st September. There is no option coming to change the date even though the slots are available. I don’t want this. I want both of these applications to go together because my mother is dependent on my father so I don’t want her application to get rejected.