My gardenia was suffering this spring. I checked the pH and it was too high. I added an Epsoma sulfur based soil acidifier. It came in a bag with a blue hydrangea on the cover. My potted gardenia has never done better! The leaves are dark shiny green and it is still flowering. I have been checking and adjusting the pH on many plants. (Thank you Santa for the pH tester).
I also read of people putting some vinegar in their water for acid loving plants. I don't know how much to use, maybe a glug of vinegar in your watering can?
