Hi,
There could be a number of reasons. Never simple is it!!
You may not be absorbing - things can change, so i would get a test done - may not be definitive , but part of a picture
Get a scan done to see how thick the lining is - if it is thick consider a course of Norethisterone as a one off maybe?
A shorter cycle in the long run may be an option, but not if you don't get on with utro.
If your womb is thin or within normal limits and you have no ill effects then scan again in 4 months to see maybe how things are going.
You could ask to go to your nearest menopause clinic .
If your Gp wont book you in for a scan or send you to a clinic , write to the practice manager, as this is negligent.
The mirena is great for some but you wont know til you have it.
Firstly though have a scan and see where you are.
I do hope you get this sorted .
PMXX