This poor thing came with the new house. It was potted and looked okay, it even had several white flowers. The soil was very dry even 6 inches deep so I watered it and the problems started. Many leaves turned yellow. This was in back in September, when we were having 85 degree weather and 30% humidity in Southern California.

I recognized the leaf yellowing as overwatering (even though I had watered it two times in seven days) so I stopped watering it. The yellowing appeared to have stopped. No rain in sight so I watered again a week later. No change.

We put the plant in the ground where it's been for the past four months and it has looked pretty much like this all the time:

What is happening to this plant? How can I help it? I see a few new leaves here and there.

Below is a mandevilla that I purchased and repotted last October. It is starting to do the same as the smaller one and it has never produced a flower (it had a few flowers when I bought it).

Meanwhile, I see mandevillas all over this neighborhood with lovely deep green leaves and lots of flowers.

What am I doing wrong?

Thank you.