lift on the Sierras?

I fly a motor glider out of the San Francisco Bay Area and frequently
head over the Sierras to the good soaring. My question has to do with
lift on the Sierras themselves.

I've occasionally been able to soar on them, but most of the time find
them marginal at best. Clouds which would indicate great lift on the
mountain ranges to the east (which are more barren and therefore
better thermal generators) often have little or no lift under them.
There are exceptions and I've had some fantastic times on the Sierras
themselvess, but I'm talking about most of the time. Which leads to
three questions:

1. Have others had the same experience as me? If not, where and what
time of year?

2. I mostly fly the Sierras from Lake Tahoe down to about Bishop. Do
they generate better lift further south?

3. Why are the Sierras so much poorer soaring? I know some of it has
to do with the greater moisture content of the vegetation and soil,
but have also heard theories about the Pacific's marine influence
penetrating that far inland. Are there other theories, or even better,
established reasons?

Thanks very much.

Martin
WT