sorry to see you didn't get any response on this (doesn't seem to be a very active forum, unfortunately). I'm considering buying a/some DM Palo Verdes for my yard, so I'm also interested in the answer.

Most likely this is an irrigation problem. We, Southern CA, have had one of the driest springs on record. Buy a soil probe with a metal "step on" so you can probe the soil to about 18" from Gemplers (online). If the soil is dry you will need the step-on to get the probe deep into the soil. Check the soil moisture under the drip line. If it is dry, water deeply so the water gets to about 18"deep.

If you have a clay soil, you will have to amend it so that it drains well for the tree to survive. Add horticultural gypsum (a fertilizer and soil conditioner used in ameliorating high sodium soils and improving drainage) Kellogg brand amendment, cactus soil, and or pea gravel to improve the drainage. Do not use sand or decomposed granite. Use some Super Thrive according to directions.
tina@H2XeroLandscape.com

Here is a link that might be useful: Gempler's soil probes