You are underwatering your plant that's why. Coco coir drains well so plants are less likely to get overwatered, but it also holds onto nutrients so you use less fertilizer. When you are growing in coco you need to water plants with nutrients with 10-30% extra runoff. Continue to always adjust pH water to 5.5-6.5 before giving it to plants. When you water to runoff it stops salt build up, replaces old nutrients with rest, also make sure whole medium gets watered. After you water lift container once everyday to get a feel for weight and when it's light time to water again.