The reason your split peas are hard is that you added salt or stock to the water before they finished cooking. From your initial post, you say you've added something called "Spike seasoning". I'm guessing that's the culprit. It's probably got salt in it.
You have to cook split peas in just water for at least an hour, then stir to break them down and add any vegetables to flavor the soup. Only after another 30 minutes or so should you add salt to taste.
Starting with more water than you think you need, keeping the lid on for a bit and then stirring a lot while it reduces to the desired consistency should be all you need to do. Don't go far from it once it starts to thicken because it will need too much stirring.
You can cook split peas in a slow cooker, but I haven't found the softening very reliable. A fast boil before you start might help.
You can cook them in a pressure cooker. This will soften them in a relatively short amount of time. Another option is to soak them longer in advance of the cooking (for a day or so) in water, and baking soda...
If the peas (and beans or lentils for that matter) are old, they will not soften. It is best to buy new ones. Their shelf life is about one year. If you can, try and buy from a place with a regular turn over of stock.
It is very frustrating to find out your green or yellow split peas have not softened even after cooking for one hour. I find that if I puree them, then cook for another 30-60 minutes, the split peas will soften.
I am alarmed to see a response that says to cook split peas in a pressure cooker. I use a Presto pressure cooker for many things, but its instructions are spelled out, "DO NOT COOK SPLIT PEAS." Please follow instructions for your pressure cooker to avoid a disaster in the kitchen.
If after overnight soaking and cooking split peas for the normal time in the recipe, you might consider putting a portion into a blender or food processor to see if that gives the desired smoothness and creaminess.
You have to put them into water for a few hours before cooking ( you can even add a hint of sodium bicarbonate to the water to help). When cooking, when the water starts to boil, you have to remove the foam from the top. Then it cooks in around 10 minutes.
I obviously the best way to cook them is to soak them overnight. There is a method that you don’t have to do that with split pea so I’ve decided to meet that theory in the middle. I put the split peas in a boiling pot of water. Let it boil and then turn down the temperature and monitor it for an hour to four hours when it reaches the softness you want to start making your split pea soup with ham, of course good luck keep in mind there are some people that like their split pea on the harder side and a lot of people like to spit pee when it gets very soft and thickens the pot with your ham and other stuff. I put salt in mine, and it comes out fine, but I do it while it is cooking halfway through.
Grew up cooking split pea soup in my house. If you want the broth thick and green with the peas soft you have to boil it hard and boil it for two hours. Salt makes no difference,I always put it in first and frequently add salty ham or bacon at the beginning as well and have never had it not soften, except when I don't boil it hard at the beginning. Of course you turn down the heat after 15 minutes, but it needs to maintain the boil, so just adjust your burner accordingly. Happy eating!