Of the CIEMs I have reviewed, I have had the NT-6 reshelled since the canals were a bit too tight. I probably could have lived with them, but since I rarely use any one CIEM for an extended period of time, except when I review it, I figured I could get it fixed. I have sent two others back for refits (no need to name names) and Grzegorz sent me another 5-way which allowed me to compare that with the new one. My first custom IEM, the EM3 Pro also has a fit issue, which I now know was due to the ear impression not being taken nearly deep enough. I have built up the canals with nail polish, and lots of it!
But I do get an ideal fit most of the time. The keys are to take a good impression following how the manufacturer recommends they are taken and make sure the block is as deep in your ear as possible. Ship the impressions in a protective box, and most of the time things will work well.