Why does this happen and how do I prevent it?
Issues with plants are often caused by a lack of nutrients for them to feed from in the water. They need macro and micronutrients in order to grow, many of which are found in tap water. Therefore if you are not completing regular water changes the plants will exhaust the supply currently in the tank and will stop thriving. Essential micronutrients need to be supplied by consistently using plant foods which encourage lush plant growth whilst also helping to prevent algal blooms. Flora Boost is specially formulated without algae boosting Nitrate and Phosphate both of which are present in excess in aquariums because they are excreted in fish waste.
Plants also need good levels of light, we would suggest a 10 hours on 14 hours off cycle to mimic daylight which can be created using an Interpet Auto Timer or standard household timer.