Is there a current running through the section the ludwigia is going through? Or something causing them to bend over?
However it is quite common for stem plants like ludwigia and rotala to grow bent over. As the plant grows bigger, you won't notice as much that they are bent over. What you could do is cut right where the stem starts to bend and replant it. The cut stem may/may not grow straight. That is also a way for you to make the ludwigia grow more densely (cut and replant close together)