Propagation of Ludwigia sp. Araguaia

This is a species that is still fairly new to me.

The stems grow vertically to the surface with no side shoots. Once they hit the top I trim them and replant the tops. I leave the bottom parts in the substrate hoping they will grow and I will have more than the initial few stems I started with. This has worked for the stem plants I've grown so far.

This method is not working on this plant; at least for me. The cut bottoms never grow again and eventually melt away. The replanted tops grow fine and once they reach the water surface I repeat.

Someone please advise what I'm doing wrong.