It could be the sun or fertilizer.
If you have them in the sun they can take awhile to adapt.
The amount of fertilizer that is needed depends on what you are giving them. For example, an organic fertilizer needs to be applied every month or so, compared to a manufactured fertilizer that only needs to be applied every 3 to 4 months.
Water could also be responsible, or the unusual cold we have been having. Who knows really, just watch them and try to monitor all possibilities.
I live in So Cal myself and have lost more trees in the last 2 months than in all the past years combined!