Gardenias can be a very trying plant. I murdered several of the poor beasts before I figured them out. They like to be cool at night and in a sunny place in the day. So put them near a sunny, drafty window. It you don’t have a drafty window stick them near your door. I have them outdoors now that we have moved south and they can stand a few nights below freezing comfortably, so don’t worry about them getting cold. In the summer put them outside after last frost, in a semi-shaded area, and bring them in before first frost. If it is too cold for them they will drop their buds and leaves may be light green. So cooler but not cold.
They like water, and need water almost daily. Keep a close eye on them and make sure the soil stays moist. They will drop all their leaves at a moments notice if the soil gets dry. Luckily they will rebound and grow new leaves if you correct the situation quickly. One of the things that makes a gardenia worth the trouble is that it is a winter flowering plant, at least indoors, setting blossoms when the nights are cool. They are heavily scented and one flower will fill the house with perfume.
Gardenias need acidic soil, basic water will yellow their leaves. So when you water them add a tablespoon of apple cider or plain white vinegar to a gallon of water.
Bud drop is usually from uneven watering or warm dry house air. Keeping them near a drafty window or door helps.
If the leaves turn yellow, but the veins remain green then fertilize, these plants are heavy feeders.
I found them to be very prone to spider mites. This can usually be cured with a couple doses of insectidal soap.
Most cases of Gardenia suicide are caused by nematodes. This is a soil borne root eater and Gardenias are highly susceptible to it. Either the plant is grown on resistant root stock or it is not.