Alice, I can see why you made this a new topic instead of putting your question in the Germination thread, but depending on the answers we may eventually join it up with that thread.
Bupleurum may need low temperatures for germination. Norman Deno the famous germination researcher got excellent results for B. aureum and rotundifolium and fairly good ones for B. spinosum at 5 deg C, but poor to very poor at 21 deg. He found B. ranunculoides did better at 70 deg, though, and B. longifolium seemed to need two cycles of alternating warm and cool three-month "seasons". Make of that what you will!
(Of the particular plants you mention, our own limited experience with Phlomis purpurea in the unwatered part of our hot south-facing Greek garden is so far not very encouraging, but the plants are at least hanging on. Jasminum fruticans grows strongly, but our plant was given a good start with summer irrigation in its first three years.)