This gnarly little thing just won't seem to grow. I fertilize, amend and water but it's still stunted.
Wow, that is pretty pathetic looking. Have no idea what's wrong BUT, if it were mine.......... I would dig it up and put it back in a pot with nice rich potting soil. Check to see if the root ball was rootbound before it went in the ground and loosen up the root ball. Then I would trim it up to only a couple of branches with leaves to encourage new growth.
Once it renewed itself, then put it in the ground in another location. Any idea how acidic your soil actually is? - they like acid.
The other thing is, have you checked it for pests? If you don't mind pesticides, I would use some Bayer Advanced Tree and Shrub on it.
Forgot to mention that I have dug it up to check the rootball, and the roots look good actually. It had a few snails and aphids but I took care of them and I have been giving it regular feedings of cottonseed meal, worm gold plus, and Bayer products. Maybe I just got a "lemon" istead of a hibiscus.
Soil in Sand Diego is typically near neutral. And although they may "like acid soil," they generally do well in SoCal.
In any event, the thing looks so puny that -- if it were mine -- I'd replace it with a healthy specimen.