Who Gives A Fish Forums

Good point about algae. Also, make sure your tank's not too warm. Vallisneria prefers cooler temps, around 72–78°F. If it's higher, that could contribute to the issue.

I'd second the nutrient and light suggestions. Vallisneria is pretty hardy but can be picky about conditions. Are you doing regular water changes? Sometimes buildup of organics can cause issues even if ammonia and nitrite are zero.