I can only say things based on the appearance of your plant, maybe my "diagnostics" are wrong... The soil seems a bit dry, and they like wet soil, the place seems a bit cold, I don't know, to produce lemons you need a good warm temperature, maybe 26-27 ° celsius. Also, maybe she had consumed some of the minerals and nutrients in the soil, try to renovate organic material... if it is the only light source that plant has, I believe that it needs more light to produce lemon, maybe 6 hours of sun per day minimum. Plants indoors tend to become filthy, so maybe you have to clean the leafs to increase the photosynthesis metabolism and to "spray" some water on it every 2 or 3 days.
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The light looks underpowered for the plant's needs. Take a look here at the "lighting for indoor trees" section and compare those specs to the light you're using. I've seen a number of inexpensive "grow lights" that don't produce the necessary spectrum or lumens, so be sure to take note of the manufacturer specifications if you decide to replace your lights
That looks like a really good resource, and according to it I do have a few things wrong. Seems like the light is probably under powered, although its hard to tell since I can't find the specs on it anywhere (GE model #93101232). So looks like I'll have to upgrade the light. I also put stones in the bottom of the pot and that link says specifically not to do that, I wonder how bad that actually is since it would be kind of a pain to get them out now