If you could post a picture of the plant that would be really helpful. Gardenias need to be in bright shade here in FL, especially in the afternoons, and when they are in pots. Some morning sun is fine, but take a look at how much light it's getting. Our spring weather is pretty brutal, dry, hot and very sunny.
Tips of leaves burning is usually an indication of too much fertilizer all at once. Try watering thoroughly a little more often on warm days to wash through some of the soluble fertilizer. We've had it get really warm, then cool off which might have made the plant take up a lot of the fert in the new potting soil quite quickly, then the cool nights slowed down the growth again, causing the tips to burn. Once the fert in the new potting mix is used up (usually @ 3 months) use a slow-release pelleted fert that is specific to acid-loving plants.
The less robust flowers are probably the result of being in the too-small pot for too long, or if it was budding when you re-potted that could be it, too.
