I'm not a big gardenia grower, but it sounds like the soil is lacking in some nutrients. If the plant is rootbound, ie, there are some roots growing out of the drainage holes, it may be time to pot the plant up into the next size pot. You can buy potting mix specifically for acid lovers like gardenia and azaleas.

Otherwise, it probably requires fertilizing. Best way is to fully submerge the pot in water for an hour, to ensure the soil is not water repellant, and then, after removing, fertilize with a liquid fert for quick results. You could also add a slow release fert later.

There is also the possible problem that the pot is too wet and water isn't draining well enough. Is this likely?

* * * *

The secret is the soil.