Hygrophila corymbosa angustifolia-this is a great stem plant-I have lots of them in my soil based tanks-both this variety and the broad leaf one. Sometimes they are grown out of the water at the nursery and the leaves have to transition from emersed/submersed growth. Its not uncommon to see all the leaves die off, drop as the new underwater leaves start to grow.
Since its potted-did you take it out of the pot and remove any rockwool around the roots-If not, I would recommend that you do that.
As the other posters mentioned-lights are a big factor with successful planted tanks and plant growth. Without the proper color temp lights the plant ferts are useless and will only feed algae. The plant can't see the light for photosynthesis it can't use the ferts.
The bulbs I like to use are-"Daylight" 6500k 20-40w depending on the tank size. I get my florescent bulbs in the lighting dept not the aquarium dept of most hardware type stores and chain stores like wal mart.
Proper color temp lights are in the kelvin of 5000-7000k with 6500k being ideal and closest to the natural sun. The florescent bulbs can lose intensity over time and it is recommend that the bulbs be changed every 12 months....you can see the light but the plants can't to use for energy.
Photoperiod is also important-10-12h/day is recommend. In nature plants get 12h of light and 12h of dark-with an hour before and after of lower light as in the sunset/sunrise-Making a good starting point of 10h/day in the home aquarium to support plant growth. Too short, poor intensity, wrong color temp can sometimes trick the plant into thinking its a season change and either go dormant, die or flower and it can also cause algae problems.
With planted tanks its all about the balance-you have to support the active plant growth so the plants can out compete the algae for light/nutrients.
What I would recommend now-remove it from the pot if you haven't already and plant the roots, make a 50% water only change to get some of the ferts out so you don't cause an algae outbreak, get the proper light bulb in the 6500k range that is under 12 months of age, get on a 10h/day PP for now. If you are using a hood-check the partition between the light bulb and water to ensure it is free of debris or remove it all together to get better light penetration to the plant......And wait...the hardest part especially if it is going through a leaf transition phase.