Try identifying the plants first maybe? They all have different needs and pinpointing what that might be should help. Some don't like to be watered that much, some need more. Yellowing of leaves could mean lack of chelated iron or magnesium even, but also waterlogged roots etc. Get a better pic of the plant/leaf then we can start identifying. :)

Its a Pittosporum Tenuifolium (Silver Sheen). Here is another picture.

Here's another close up picture of leaves. Sorry, I couldn't figure out how to post more than one image in a single post.

I searched around and found two pages that might be helpful...
http://ufdc.ufl.edu/UF00065943/00001/2j

http://www.mygardenguide.com/plant_detail.html?id=10711

looking the plant up (I'm a novice, so take anything I say with a grain of salt) it says the plant needs acidic soil. You don't say where you are, but in the Southwest, the soil is generally alkaline, so that could be a problem.

or it could just be heat stress this summer