I'm no expert on hibiscus but it looks like some nitrogen might help it? Although in my experience when it's a nitrogen issue, it's just the new leaves that are turning yellow. Another possibility is that it might be is the soil around it is retaining too much water (this can change even in a couple feet). How are you watering it? You could try reducing the flow of water it receives, but extend the amount of time. If it's a 1-2 gph emitter, change to .5 gph emitter. If you're using a hose, maybe slow the water a lot (just barely dripping out of the hose).

I have read that hibiscus do not like to be fertilized. I haven't had much luck with hibiscus myself, but I have seen this same thing happen many times- one plant is deliriously happy, and the one next to it dies. There doesn't seem to be any reason for it, I don't know if they get a disease or if there's an insect that will hit one and not another. It just seems like if they're happy they do great, and if not, there isn't much you can do for them.

It looks stressed, since it's next to a wall and rocks (both which retain heat), I'd suggest moving the rocks to the side and putting down composted mulch around it and then use a good fertilizer. Check the soil by digging down a couple inches and see if it's dry or wet and adjust watering if needed.

Kevin

Could be iron deficiency especially if the leaves are yellow with green veins . More common in higher alkaline soils like the desert.