Q.Hibiscus bush not getting bigger and stopped flowering
I planted a Hibiscus bush about 3 months ago. It seems healthy, but it’s not getting any bigger and has stopped flowering. What can I do to help it along? My soil is very hard clay, in Southern California.

Since you just planted the hibiscus not that long ago and it seems healthy, I wouldn't worry too much yet. It probably still is experiencing growth underground (roots) and will soon generate new foliage and stems. A new bloom cycle should re-establish itself.
However, it wouldn't hurt to review the care of hibiscus and make sure you are watering and fertilizing it according to its needs. We have an article that discusses this in detail here:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm