Guys I'm starting to think my fit isn't good. The sound changes a lot when I move my jaw around. It sounds most clear when my jaw is clenched and it sounds more muffled when I open my jaw slightly. My natural resting position of my jaw is slightly open, not clenched. Is this normal?
You might benefit from a re-fit, but you should also be aware that some people have ear canals which
vary to a greater extent than other people.
If you have ear canals that vary in size and shape (depending on what position your jaw is in),
more than most people, then you may need to balance your priorities - if you go for a slightly larger CIEM shell (in the ear canal section of the CIEM, I mean), then you may get better isolation in various jaw positions, but you might reduce the overall level of comfort, which could, potentially, annoy you much more.
However, we do not know if you are one of those people who have an above-average amount of variation in their ear canal size/shape in relation to different jaw positions. Because of this, it may be worth at least getting new impressions done, and asking for a refit. Otherwise, you may never know.
"If nothing ventured, nothing gained..."
There is some related discussion here (I have no affiliation with Noble - it's simply that I remember contributing to the following posts, and therefore know that they are relevant, and where to find them):
http://www.head-fi.org/t/684787/noble-audio-the-wizard-returns/4125#post_10560161
http://www.head-fi.org/t/684787/noble-audio-the-wizard-returns/4125#post_10560660
http://www.head-fi.org/t/684787/noble-audio-the-wizard-returns/4125#post_10560187
and, if you haven't yet seen it, there is lots of useful discussion about how to increase your chances of getting the most worthwhile impressions, here:
http://www.head-fi.org/t/578855/things-to-consider-before-getting-your-custom-iem-impressions-done-the-perfect-fit
Please note that in the above thread, I emphasised the importance of being very careful to not accidentally clench your jaw when having impressions done. When using a bite block to open the jaw a little, during the impressions procedure, it can take a very conscious effort to avoid tension in the jaw muscles, because the jaw is not accustomed to remaining open for several minutes at a time. As daft as it may seem, I recommend you practice keeping your jaw muscles relaxed with a bite block in-situ, in the days before having the impressions done.
Good luck, and I hope it all turns out well for you