Over fertilize?

I have several gardenia bushes among other shrubs in my southern california garden.The other shrubs look healthy but the gardenia leaves are a pale green, darker green veins, brown edges. Soil pH was 8 so I added mulch,15-15-15 fertilizer and sulfur at different times. No improvement. Is the problem too much fertilizer? Suggestions?

Gardenia's need Magnezium ...for green up or food for acid loving plants, never heard of sulfur for them?

I imagine it was probably the PH as much as anything else, 8 is really alkaline, most plants won't like that.