Is it in a pot? In the ground? What type of fertilizer did you use? What type of water are you using? Wellwater? Next, what type of soil. Sun? Shade? Then, is it grafted?
kevin

Kevin thank you for responding I was using a fertizer /acidic says its designed for gardenias. I have well water. The plant is in the sun/partial shade part of the day and its on the west side of the house. Any ofthis help trouble shoot?
Tanya

Yellowing could be a sign of needing an iron supplement. I have the same problem with gardenias, altheas and some of my roses. I use Ironite and the leaves green up real well.

Thank you very much Ill try that .And yes as a matter of fact I did have a rose that was yellowing
Tanya01

Gardenias often suffer from lack of magnesium, which will cause yellowing of the leaves. Use 1 tablespoon per foot of height of epsum salts (magnesium sulphate). You can sprinkle it on the ground around the drip line or dissolve it in water a pour around the drip line.