1. Before making a decision on fit for your IEM's (JH13's, UE10's, etc), please remember to have a clean set of ears. To do this, in the shower turn your head to each side (right ear on your left shoulder and vice versa), then allow the water to enter your ear canal. Let it sit for a few seconds, then dump it out. Do this 3 times for each ear every day or other day. This helps flush out sediment and built up perspiration mixed with wax. Use a Q-Tip to carefully dry out the ear canals after showering.
I found this GREATLY helps in getting the best seal possible and in determining if your pair needs refit. (also aids improving seal when people are turning their heads, opening mouths wide and eating)
2. Sometimes a person's hearing can come into play as well. In that, if one ear hears better than the other, regardless of fit, one may be able to detect more ambient sound through the better hearing ear. This info could be useful if the fit seems good for each ear but one side "seems" to let more sound in.
This story is just some additional food for thought on the fit and potential refitting of customs.
When I received my UE11's, I thought for sure the right ear needed refit. Such that, I went to the lab in Irvine for examination and their technician did another impression and tweaked the 'R' monitor onsite. He removed some of the tip of the barrel in hopes of promoting a better seal and rescanned the monitor for their records. Seeing is how the UE technician's fit ratio was over some 96%, I was confident about the refit.
After coming home and settling in, the right side still felt off. So I then went back to the audiologist in Phx and had two additional impressions done for my right side. (I had two impressions remade to help offer the best specimen for their lab to forge a refit for the refit). I sent the then back to UE for work on the right side.
After several refits, the right side on the UE11 still does not "seem" to be as fitting as the left. Only now I have some "scar tissue" in the form of extra material on the tip of the right side. (I did not like that the left barrel was longer than the right barrel, and since it was being refit, I requested the barrels be made the same length again) The additional fit is the same as it was from the beginning, and the UE scans were nearly identical Per the UE lab for each of the 3 times they worked on them.
I want to emphasize having clean ears to start with for the best sound, fit, and knowing about the fitting nature of your customs. I was never aware of the trick above from No 1 until last year.. (that you could tilt your head and allow water to enter your ear canal), my audiologist helped suggest this. If I had known this before making the judgement on my UE11's Right side not fitting, I would have done this first. Because, as it stands, the UE11's fit the same as they did when I first received them, and the lab's multiple scans of my 'R' impressions suggest they were correct perhaps the first time (and maybe all along). I may have never needed the refit(s) had I cleaned out my ears better. (note: I thought they were clean then too) Too often we strive for perfection in sound and fit, me included. This is why I am sharing, it may help as a step for new owners, and to save time and money. (especially overseas listeners)
After that pair, I have never had a problem with any other customs fitting thus since, (counting UE10, JH13, & JH11).
Hope this helps!!