There are some types of leafy vegetables that become bitter when the temperatures rise. Spinach, kale, and lettuce are among these. Try to plant them in an area that stays cooler and harvest them when it is cool.
Try doing a spring and a fall planting of the cool season vegetables to get better food and avoid the bitter taste.
These and other plants and herbs also go bitter if they go to seed. So make sure you harvest before the plants go to seed.