Potatoes that won't cook
The phenomena of potatoes that won't cook has happened to me a few times now over the years, yesterday, it happened in a beef stew I was making for 50, I had started cooking it the night before, then refrigerated it till morning then started it cooking again, planning for another three hour of slow cooking. They never cooked. I thought perhaps there wasn't enough liquids and added hot water, that seemed to help, but when they had been simmering in the stew for a period of 6 hours, I gave up and took them out of the stew to serve it, thankfully, it was delicious without them.
But what is up with this?
There has to be some food science behind this.
LOL, I know exactly what happened. You're "pre-cooking" of the potatoes set the starch on your potatoes. Once that happens, potatoes can be very hard to cook. This precooking is usually a technique done for mashed potatoes, because since you set the starch on your potatoes, your mash won't get gummy. Chefs at top restaurants do this and pass the mash through a sieve plenty of times to get an extra fluffy and velvety texture without making the mash gummy. Pre-cooking potatoes at about 68C will do it for you. I've done this for mash and have had to use a pressure cooker to cook the potatoes, as it takes too long to do it traditionally. It's a great technique for extra fluffy mashed potatoes.
In your case of cooking stew, just don't pre-cook your potatoes, and I definitely think you want boiling water to cook them. I'm always surprised at how long it takes a normal potato to cook through.
Perhaps your potato problem is the starch that is pulled from the potatoes when heated. Slight heating will pull the starch, but not push it away from the spud. The starch is the white foam seen when boiling potatoes for mashed. When the potato cools, the starch (being sticky) adheres back to the surface of the potato, "protecting" it from further cooking, or just making it take longer. Next time try one of the following methods: soak the potatoes for 4 - 6 hours, then add to the stew for the last hour or so. Or, boil until *mostly* cooked (firm resistance in center when pierced with a knife), then add to the stew (if you calculate the water just right, you end up with starchy potato water you can add as flavoring and thickener to the stew). Also, consider using red potatoes, since they have less starch than Yukon, Russet or Whites. Let me know how it goes.
I wonder if the water ever got close to boiling. Meat will cook at lower temperatures, (steaks are brought to 135, slow cooked braises to 180). But many vegetables take much longer to cook when temperatures drop below sea level boiling. People moving to high altitudes (above the 5000 ft of Denver) often encounter this problem.