I have found it helps to give it regular fertilizer for acid-loving plants. I use the water soluble MiracleGro.

Needs acid compost or soil - if in a pot, use ericaceous compost and follow Wylieinthea...'s advice above.

My gardenia did this. It was outgrowing the pot it was in when I bought it and I repotted it using normal compost and it turned yellow. After learning the error of my ways (and being a bit annoyed at having to by 25L for a single houseplant :) ) it seems to be perking up fine.

Welcome, Harrisonkim!

If the new leaves are turning light greenish yellow with green veins, that means that the soil is too alkaline, and you will need to use an acid forming fertilizer, and also an acidic mulch such as coarse compost or cedar bark.

If all of the leaves are turning an even, tired, yellowish green, then that usually means that it needs nitrogen, which an acid forming fertilizer will also fix.

If the older leaves are turning a bright chrome yellow and dropping, that usually means a watering problem:

Underwatering, which causes the plant to drop the older leaves to prevent dehydration. That happens when the plant wilts between soakings.

Overwatering, which damages the roots, and means less water to the leaves. That doesn't show wilting at first, but at its worst, it can throw the plant into a permanent wilt and death as the roots die.

Irregular watering, which buys you the worst of both worlds.

I hope that helps some!