Buy a bronze watering can, select it and tap the flower you want to water twice. As for the fretilizer you only need it when watering a flower doesn't work.

Close the game and re-open it. There will be more bugs near the flowers.

Just plant as many veggies as you can in the garden, you don't need the fruit trees.

I plant the vegetables in the garden and the flowers in the green place in front of the museum and the store.

Go to the store and buy Tier Fertilizer (there are 3 different ones, buy whichever you like) from the plant section and use on the flower. It will definetly help.