Kushal,

How much sun is that getting? They don't like afternoon hot sun.

Your gardenia has chloric leaves, which means it needs a little bit of Epson salts in the water when you water it, til it greens up a bit.

You don't have to polinate it, the bugs will do it. Trust me, they will find it when it blooms, it will give off fragrance and they will fly up to your balcony.

I prefer slow release osmocote, hard/impossible to over fertilize. Also how much/little water are you giving it? Take a look at the roots if you can pull it out and see if they're rootbound or too wet which can mask nutrient deficiency. i.e. if they're watered too much they may not be able to utilize the nutrients.