Length of lighting period is a big factor. Some plants will just grow at a certain length of time but with more light they produce their fruit. The fruit of aquarium plants should be a flower but I'm not sure that all aquatic plants do flower. What brand or type of light are you using and what type bulbs? What spectrum ? More red than blue/ daylight or equal or what? Old time indoor plant growers used a simple incandescent fixture with what was known as a Gro-Lux bulb and had great results. Reveal your secrets !