If the plants are new to the tank, then it can take weeks for new plants to get used to your tank conditions. Some plants will not. You have decent lighting, so all you need to do is keep the water extremely clean and change half the water in the tank every week.
Weekly, large water changes replace nitrates, phosphates and sulfates the plants need for healthy growth. You could also add a good liquid fert a couple of times a week. Tetra's "Flora Pride" is a good one. Just a little is sufficient.
Another good thing for your plants is to feed your fish a healthy diet. If you feed a variety of frozen foods a couple of times a week and avoid the freeze dried and flake stuff, your fish will provide a good fert source for the plants too.
A couple of thoughts for you.
"Fear not my child, just change the tank water."