For osteoporosis prevention, the serum estradiol to aim for is 400pmol+ (according to Dr Newson in a recent podcast episode).
Sadly your experience is extremely common. Women take HRT and think it is a black and white thing - they are either 'taking' it or not. And if they are taking it, they assume they have all the benefits which come with it.
Really, the benefits of estrogen are dose dependent. If you are taking enough estrogen, you will probably have a bleed - because it will be causing your endometrial lining to thicken. If you're post-menopausal and not having a bleed and on continuous progesterone, then it's likely you are also not getting enough estrogen to protect bones, heart, brain etc. Doctors are very afraid of estrogen - still - and will typically be looking to reduce it, always.
It shouldn't be this way but women need to educate themselves, request blood tests, and ensure their estradiol is at a therapeutic dose - and advocate for increases if it isn't.