Plants need a slow steady supply of food (plant fertiliser). If you don't give them any for a while they starve and when you do eventually give them some, they suck it all up quickly (within a day or two). Then they starve again. This cycle of food and starvation is bad for plants and stops them growing properly or at all.
Liquid plant fertilisers remain in the water until used by plants or algae, or until they are diluted out with water changes.
If you are using a liquid aquarium plant fertiliser, you should monitor the main ingredient with a test kit. Most are iron based and you check the iron levels in the water with an iron (Fe) test kit. Keep iron levels at 1mg/Ltr (1ppm).
When you first start adding a liquid aquarium plant fertiliser, the plants will normally use it all up within a day or two of being added to the tank. So a lot of people will add a half dose of fertiliser every couple of days for the first few weeks. Then they add it twice a week for a few weeks, and then a full dose once or twice a week after that. This allows the plants to take up the nutrients and get to a point where they have drawn in enough fertiliser to keep them going and they don't remove it all in a day.
Whenever adding aquarium plant fertiliser to aquariums, you should always do a huge water change every week to dilute any excess nutrients from the water. This prevents overdosing if the plants have not used up all of the nutrients before you add more.