When you initially insert the JH13, try seating the shells a little less tightly in your ear. Having owned many customs over the years, I found that when inserting them, they should make just enough contact with the outer ear canal and bowl of your ear to form a seal. The shells shouldn't be excessively pushed in or be forced to seal. A merely requisite seal allows the ear tissue to heat up, expand, and naturally seal with the custom's acrylic (or silicone). Twisting an IEM too tightly into your ear may yield an immediate seal.. but over time (i.e. in your case, an hour), the excessive pressure doesn't let the ear tissue form a "natural" bond with the shell of the IEM.. leading to pain and fatigue.
I typically twist my JH13 in just enough to create a "loose" seal (as I previously described), then I grip the two "corners" (at the top and bottom of the shell) and very gently 'jiggle' the shells up and down for a few seconds to stimulate the inner ear tissue. That's it. Within thirty seconds, a natural seal has formed and the JH13 feels virtually 'invisible' in my ear. This has just been my experience, but I found approaching cIEM insertion this way has consistently yielded a comfortable fit for long periods of time.. and the SQ is delivered as intended.