The problem is that there is a tall plant in what looks like a small bucket of some type of potting soil mix. The bigger the plant the more it will need. Since the roots cannot expand out past the sides of the bucket they will not be able to find the raw nutrients the plant needs except for what is in the bucket.
On top of that, once the plant starts to go into flower it needs even more nutrients than it was needed before when it was just growing.
advanced chemical fertilizer"? Whatever it is, follow the instructions on the bottle or bag of fertilizer. Not only the amounts of fertilizer to amounts of water but also follow what is says about how often to give the fertilizer.
soil light mix" that you mention? There should be a company name and then the product name. Then somewhere on the bag should be important info such as what the soil mix is made out of and sometimes the basic ratio of ingredients.
Most likely the problem is that the soil mix is not able to supply the amounts of Nitrogen, Phosphorous and Potassium the plant needs at this stage. Everything that the roots can get are being used for photosynthesis and for the flower development And, it will get worse because as the flower buds get larger the plant needs even more than the day before. The group is making some excellent suggestions so follow along.
As far as I can see it is best to continue with the advanced chemical fertilizer and use the nitrogen, phosphorous and potassium as a bonus. Things to keep in mind include that at this stage the plant has slowed down the growth of new roots so do as little as possible to damage what is already there. Also, the plant has pretty much stopped the growth of new large fan leaves--it is growing new sugar leaves and small fan leaves but those big ones that are used for photosynthesis and nutrient & sucrose storage are what you see.
Do not cut off any of the leaves that are turning yellow until they are completely yellow just like the yellow on a traffic signal. Better yet, let them fall off on their own.