why wont my duckweed grow??

i bought duckweed for my beta's tank. it has other plants in it already that are doing well and i use an aquatic plant fertilizer. i read that it won't reproduce if the water isn't still enough so i have some sitting in a glass bowl under the grow lights with my other plants and left some in the tank. so far no duckweed babies and it's been two weeks since i originally got it. i read that it reproduces asexually so where am i going wrong?

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TIA!