Lack of Sunlight
The reason any of your plants begin to look leggy is most commonly due to a lack of adequate light, and in turn growing extra-long stems in order to reach towards the light. If your euonymus plant is not receiving enough sunlight, it will start to put more energy into growing longer stems. This can slow down leaf growth and cause the plant to look leggy, sparse, and not the healthiest. Your euonymus prefers indirect but bright light. Bright indirect light can be found in places close to an east-facing window or a few feet back from an unobstructed southern or western window. If the southern or western window has something like a sheer curtain, natural shade from a tree or building outside, the plant can be placed a little closer.
In addition to giving the euonymus more sunlight, you can prune the leggy branches away. This promotes new growth, restoring the euonymus with lush, full foliage. If new growth is coming from the plant base, trim the tall spindly stems above a leaf node to allow more light to get through to the new growth. Your plant should respond by sending out new growth by the node.