Which is more important in tea: Leaf or Whole Branch/Stem?

So lately I've been drinking parsley tea to regulate my period. Earlier when I've used it, I usually throw in a whole bunch of it including the stems and leaves. But last week I only tore off the leaves and threw them into the tea but it hasn't given me my period at all yet. I was wondering what the difference was and if the stems were more important than the leaves.