I think I've (mostly) figured it out.
I notice that I can achieve the desired seal when I initially push in my triple flange ER4XR's. However, after a minute or so, the IEM's tend to slip out and lose the "perfect" seal (it still isolates very well in this state but not "as well"). For example, if I do the "finger rubbing" isolation test when I initially seal them, I can't hear a thing. However, after a minute, if I retry, I can just barely, barely hear it. Is there any way to fix it? I'm using the large tri flange tips, as the small ones don't get a seal in the first place.