Hmm, it's possible they may be old. The older dried pulses get, the tougher they become and consequently the longer they take to cook.
But to cut to the chase, I wouldn't use a slow cooker at all. They need fast boiling in plenty of water for fifteen minutes then moderately fast simmering for up to another hour maybe, rather than long slow low temperature cooking. Pre-soak them all day/all night before cooking. In fact if possible pressure cook them in a pressure cooker for thirty to forty minutes. You should be fine. As for flavourings a straightforward 'garam masala' mix should be ideal. The cooked peas & garam masala mix added and simmered into a veggie base 'sauce' of tinned tomatoes, fried onions and / or spinach, served with rice or flat breads would be lovely.