In the first case, we may experience this when we start a new hrt: we feel improved estrogen uptake for a few days, and then our improvement seems to evaporate.
What's most likely going on is that the situation of poor coverage before beginning the new hrt/adjustment had our system stressed. This means that we were borrowing from other systems to prop up our estrogen levels, the usual technique for meeting high priority needs from lower-priority resources. When we then add more estrogen into the situation, we feel that boost initially. But pretty soon word gets out and all those other systems that were being shorted start taking that loaned-out support back. So what was a comfortable supply suddenly is less so.
That doesn't mean we're worse off: we're actually in better shape than we were because we're operating under less stress. But it does mean that we probably need another upward dose tweak once we've given things some weeks to settle in.
Weeks? Yes, because this take-back process isn't a single one: it happens over 6-8 weeks in a series of small iterations. This is one of the arguments for the "take it slow" aspect of gentle-on-the-body hrt tuning. There's no sense to tweaking (by which we mean fine-tuning; obviously if something's terribly off, we need to intervene before that) before we're settled in and all of these readjustment processes have played out. Why not? Because that just involves a long chase and a real risk of dosing ourselves up into excess. It's clearer to wait until we can see the target before we try to reach it.
The other situation in which we suddenly feel as though we're not getting what we used to out of our hrt happens months or years along in hrt use. We felt mostly fine, but now we're clearly not. Did our hrt "stop working" in this instance? Nope.
In fact, in this kind of situation the element that is most likely to have changed is us, not our hrt. There are lots of things that might have this effect. Maybe
- we moved to a new location with new exposure to environmental estrogens
- we changed our diet (becoming vegetarian or vice versa can have a big impact)
- we changed jobs such that we're eating lunch in a different part of town/different restaurants
- we changed supermarkets
- our supermarket changed distributors
- some sort of unreported toxic spill happened
- our stress level changed significantly
- we started or stopped drinking or smoking
- we lost or gained a substantial amount of weight
- we took up or quit an exercise program
- we changed our brand of sunscreen
- our pharmacist or insurance company changed us to a new generic or a different hrt altogether
- climate change means warmer weather that may be affecting shipping or wearing our hrt
All of these things and more are possible disruptors of longstanding hormone balance but because we don't think of them as hormonal factors, the changes they represent are essentially invisible in terms of how we think about our hrts. But none of them represent hrt failure: they just mean that we need to tweak our dose a bit. And that's an important distinction to keep in mind.