The girls are out to drive me crazy

Ok how can 6 Alpacas all hold off on having their cria.  I have 5 girls who are all over due and one who is due now.  I am watching and watching till they are sick of me looking underneather and watching them nap. I see them laying on their side and go and watch them.  A week ago one of the girls fooled me and she was my last one due and she gave birth to a gorgeous little boy. What gives there is no rhyme or reason to them. I swaer the laydown and they take bets to see how long it will be before I am checking them.   I think when they deciede it is time they will come like popcorn or should I say papcacorn