Why is castor oil nor mineral oil working to induce my labor?

I am 38 wks and have been having contractions since I was 34 wks. These are not BH and I am still 1cm dialated. I was told to try castor oil, and I did. At 37 1/2 wks I took 4 tbl spoons of castor oil and it did not work, not even diarrhea. And last night I took 4 tbl spoons of mineral oil and again nothing. No contractions and no diarrhea. I already know the side effects for all the know it alls. So am I taking the right dosage or do I need to take it more than once?