Do both of you take your replacement hormone pill at least an hour before eating in the morning so that there aren’t issues with some of it not being absorbed properly? Some foods and vitamins can be problematic, especially iron and antacids. I believe oatmeal is another. If you can take vitamins and other meds several hours later you are probably safe.
Also, are you certain that you’re not forgetting to take the pill some days? That’s very easy to do. If this is a possibility, then I suggest you get a pill box. That way you can tell if you’ve taken it because when you have, the pill will be gone.
Those are some common reasons for fluctuating levels.