What type of natural preservatives do you use to protect your soaps. Right now I am not using any and some soaps go rancid and some do not, usually the light colored ones.
If made properly and saponified fully, soap will last several years. I am sure you are making it properly so I imagine it is oxidation. Some oils and EOs are prone to this.
Using a good food grade sodium hydroxide is important for consistency. Of course you do need fresh oils but I have kept olive oil for two years with no difficulty. There are different compounds you can try. We used to use Rosemary Extract.
Contact: Rockland Foods in NY, 914-358-8600 – ask for information on Stabil enhance-OSR liquid product #2601. They should be able to email you some technical information. I forget what the cost is. It has been several years but a gallon lasted us about three years.
We used 11 grams per 54 lbs of oil. Very little is needed. You can also put it in your olive oil to get more life out of it. This does make a slightly darker shade on your soap even though it is a tiny amount. You want to shake it when you use it and keep it in a dark place. Every few months warm it to about 120F, then shake it up.
Vitamin E (natural) helps but is expensive. There are other products that are available.