I've been taking primrose orally since 34 weeks and vaginally since 36 weeks. I've tried everything else too. Didn't stop working until 39 weeks with preschool kids (I stayed very active), walking (over 2 mil at a time), bouncing on a ball, raspberry leaf tea, pineapple, Clary Sage when I do have contractions, ankle massage, etc. Nothing more natural has worked. I promise, castor oil isn't my first option.
Google it and you'll see more information listed about how it works.
Dr's say that it isn't safe to use castor oil. Try Primrose oil capsules. They're available at Walgreen's. You just poke a hole in them and insert the capsule into your vagina to soften your cervix and help you dilate. Do it 3 times a day and especially over night. Dress yourself with a panty liner because the oil does drain out. Ive used it with my son just 3 days before my due date and went into labor on his due date but had him that following day. It's natural and safe.