Don't give up yet. The plant she sold you may not have been suitable for
your tank ? Just because it grows like a weed for some, doesn't mean, it
grows under your lighting, waterhardness, fertilizers (the ones coming for
free with tap water included) etc.

I for one cannot keep Duckweed alive, no matter how hard I try, but all
other plants are doing fine.

I think a good start is, what kind of fish do you have ? What's you water
hardness and temperature, and what's your lighting like ?

I am sure we can go from there.

Greetings
Mad