Ludwigia is not an easy plant. There is at least one other thread here on problems growing it, with a couple responses from different members having similar problems. I don't know which exact species you might have (there are several), but only one,
Ludwigia repens, is relatively "easy" to maintain in aquaria. It tolerates soft or hard water, moderate light, but requires good nutrients. The leaves are olive green to reddish, underside green or wine red.
All other species, including the bright red varieties, require soft water, high light, and good nutrients.
Having said that, all stem plants will frequently lose lower leaves, primarily due to less light reaching them. Increasing the tank lighting is not always the answer, since lower leaves are naturally going to be somewhat shaded by the normal growth of stem plants. The options are to plant other plants in front that will remain low and hide the bare lower stems, or plant the stem plant behind objects like wood or rock.
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