I have a hill that goes around the front of my house where I am trying to plant easy-to-grow plants like a white sage or purple hopseed bush. These plants are supposed to be easy to grow (I've grown them before with success), however, the plants are not growing well here at all. Obviously it's the soil (they've been getting enough water). What can I do to do the soil in order to grow plants there? Should I add mulch, compost, send a soil sample somewhere?? Any advice would be much appreciated. The location is the Los Angeles area, zone 10.

Yes, step 1 is a soil test. LA County Extension is a good place to start. Adding compost/mulch can only help. If you're near an Armstrong Garden Center there should be someone there that can help.

Are the roots white or brown ? If checking the roots they are white the plants is still good. Add a soaker hose to the area on a timer to water. The soil there may not be draining well and to test the drainage, by digging a hole in the soil near the area where the plants are, then flood the hole with water. Check for how long it takes for the water to run thru the bottom of the plant hole. If it not draining well the soil needs to be upgraded to drain better. Roots don't like to sit in water. Or the soil may be draining to fast for the plant to absorb enough water. Need Amendment's for soil improvements .. See soil amendments for your type of soil.