Anubias nana is a slow-growing species making it one of the easiest plants to maintain.
It will only need trimming every now and then (when the stems get too long).
You need to keep the tank clean, or the buildup of pollutants will gradually become toxic and kill any life in your tank. Regular water changes (every couple of weeks) will control nitrate levels.
Cleaning the tank keeps the water clear too. This makes light more accessible to your plants for photosynthesis.
As the plant grows it takes in nutrients from the water. If there aren’t enough nutrients, its growth will slow down.
If you think this is why your plants aren’t growing then there are plenty of nutrient supplements that you can buy in stores. Another option is to use nutrient-rich substrates. Just be careful that you aren’t adding too many nutrients or you could encourage algae growth.
You can add CO2 for the same effect (since it’s taken in during photosynthesis), but Anubias nana manages with the CO2 expelled by fish as they respire. Adding too much CO2 will harm your fish.