sounds like you're trying for campsites in high season.
that's why we refuse to camp in California in the summer.
we have no problems getting Yosemite sites for April-May, but you still need to start reserving when they are first available.
like it was said, too many people wanting to camp in California, for the amount of campsites available, especially coastal campgrounds and Nat. Park sites.
remember, you're competing with not only Californians, but out-of-staters and visitors from other countries.
that's why we camp in the spring, fall and winter. since you can camp year-round in California, why do it when everyone else is.
the weather is still great, especially in spring and fall. Tahoe is great, in the fall after the kids go back to school.
of course, if you have kids, then you're stuck.