There might have been too much moisture in the slow cooker, or you cooked the peppers for too long. Only pour ⅓ cup of water into the slow cooker, even if it does not look like a lot. The peppers will release moisture as they cook so you do not need any more than that. Also be aware of how long they are cooking. If you set the temperature to high, try not to cook for longer than 3 to 4 hours. If you set the temperature to low, try not to cook for more than 4 to 5 hours. Keeping them in there longer than needed will cause them to become too soft.