Hi HaRuHaRu
I am assuming from your comments that you are on HRT - because you said "if your bleed is light does that mean the progesterone is working?".
Progesterone does not prepare the womb for a bleed.
During the natural fertile time in a woman's life, oestrogen from developing egg follicles in the ovary stimulates the womb lining to grow - until a mature egg is released "pop" . It travels down the fallopian tubes to the womb, "hoping" to meet a sperm. If it does then it implants into the womb and you are pregnant - and other processes take over (to increase oestrogen and progesterone).
Meanwhile back at the ranch (the ovary) the empty egg sac ( called the corpus luteum) starts pumping out progesterone to get ready for the developing embryo and it does this for just under a couple of weeks ( not sure exactly) The progesterone CHANGES the structure of the womb lining so that it can be receptive to a fertilised egg. Egg meets no sperm so no implantation and no pregnancy therefore the other processes don't happen This signals to the body - not fertilisation etc so as the corpus luteum in the ovary dies, the progesterone level in the body falls, and this signals that the womb lining is not needed, so it is shed.
I've said all that to help understand what is going on.
When on HRT, and taking oestrogen for our well-being and various reasons - a by-product of this is that the womb lining is also stimulated artificially so we need to shed it artificially. That is why we take progesterone. Firstly when taking it cyclically it acts just like in our natural cycle - it changes the structure of the lining, and as with the natural cycle, when you stop taking the progesteronem the level falls in the body and the lining is shed.
As joziel said this is dose dependent - more oestrogen = thicker lining therefore more progesterone needed to shed it all and as she says, a light bleed can mean all sorts of things. The most important thing is that the bleeding is normal. If it comes at approx the right time after withdrawal of the progesterone or just before - and if you are post-menopausal - then this is normal. Bleeding at other times or very heavy unexpected bleeds may be abnormal.
If you still have a cycle then this can lead to bleeding at the "wrong" time , though not abnormally.
Again things like fibroids or polyps can also cause abnormal bleeding.
If you are post-menopausal and taking continuous combined HRT then progesterone works slightly differently - it eventually interferes with the oestrogen receptors and some how prevents thickening of the womb so it never build up.
The half-way scenario - eg taking progesterone 21 days out of 28 - I have no idea what happens - but probably a bit of both - build-up, prevention of build-up, and then shedding?
Oh dear I seem to have written an essay but I hope some of it helps explain your confusion. Do keep on asking if not! We're here to help!
Hurdity x