Hi Dazned,
I've been on 25mcg Estradot and 10mg MPA tablets (monthly) since the beginning of February. I had two withdrawal bleeds when I expected them and none for the past two cycles. If you look in the menu section > Treatments > HRT Preps > For Perimenopause it says these preps cause a withdrawal bleed in 85% of women, meaning that 15% do not get one. I never had a period on the mirena coil either. It could be that you and I are one of the 15%.
If you are worried at all, you should speak to your GP/Nurse Practitioner/Pharmacist.
Personally, I'm not worried. I had a TV scan last year for something else and the doc remarked about how thin my lining was - I wasn't surprised as I've have an ablation, so there's not much lining to stimulate.
Galadriel x