I have duckweed in all 4 of my regular tanks and 3 other containers, and each one grows differently.
-My heavily planted 10g tank has a full cover of duckweed, and seems to constantly have mature leaves, with very few baby ones (mature are about 1/2 inch diameter, babies as small as 1/8 inch). 2-15w fluorescents for lighting
-My secondary tropical tank, moderately planted, has a full cover of duckweed, but has an even mix of mature and baby leaves. Lighting is 2-10w mini-compacts
-My 5.5g betta tank has a full cover of baby duckweed leaves only, strange, I know. Lighting is 1-13w compact fluorescent, with additional ambient light from a nearby fluorescent fixture.
-My 10g goldfish tank has some duckweed, but I think the goldfish is keeping it in check, as it doesn't expand much, and doesn't seem overly healthy, but isn't brown/dying. 1-15w fluorescent plus indirect light from nearby window.
-My half-barrel pond gets my extras from the overgrowth on my other tanks, but the mature duckweed gets brown quickly, but it also constantly has baby duckweed that is pretty prolific. Lighting from direct sunlight 6-8 hours/day, bright shade the rest of the time.
-The other containers have full duckweed cover with moderately healthy duckweed, mostly mature. They get either moderate fluorescent light, and/or indirect sunlight.
My conclusion - duckweed is fairly easy to grow, but seems to better in moderate to high light and tropical-ish temperatures (75-82 degrees F). I think a lot of direct sunlight may be overkill, especially outside since most plants need to be acclimatized slowly to true sunlight and variable temperatures (Northeast Ohio, go figure). I've tried to keep my pond around 70 F , but the tops of the plant may be getting too cold at night when it drops to or below 50. Duckweed and other floating plants are better at utilizing light/sunlight than submerged plants, so they tend to do more, with less. The other part of the "aerial advantage" is being able to get nutrients from the air and water.
The point of that ridiculously long diatribe up there, is to suggest you make sure you have some moderate fluorescent lighting (1-2 watts/gallon) to get it growing, and keep temps in the tropical range, if possible, for best health. The fact that you have little baby leaves though, is a great thing. It means that the plant(s) are getting enough nutrients and light to "get it on" so-to-speak. So just be patient, it doesn't usually take over overnight, but when it does get going, it seems to grow exponentially.
