I have no problems at all with the SH build-up and was always over 10mm, which is really perfect. (3xtägl2mgEstrifam at 45kg)
But I'm really lucky and an absolutely positive example. A lot of people here have problems with it and are happy to get over 7mm ...
But 3.5mm is nothing. No TF is made with it either.
How old are you? What do you weigh?
My coordinator once said that it becomes more difficult after 40.
What else are you doing? Are you taking ASA? That helps with blood flow to the mucous membrane. I also drank pomegranate juice (Dm). I went for a lot of walks to improve circulation. Eating healthily, little sugar should help.
When was your last period? If it's been a long time since your last period, trial cycles 2-3 can help. Sometimes the body needs to shed, build up, shed,.... to form new blood vessels.