Certified GKH Gardening Expert
Snap peas need to be eaten early. Once the peas swell, completely, they will take on a bitter taste, or barely any taste, depending on the cultivar.
Sometimes, adding sulphur to the soil can help. Adding this as potassium sulfate will improve fruit set as well!
One other thing to note is temperature. Peas are a cool season crop, and heat can ruin the flavor. It has been quite warm in my area this week, and if it has been warm in yours, then this could be the reason.
Here is an article that will help you to grow snap peas: