In performing my monthly mold cleaning, I noticed that one factor that helps my Select-a-Vent inserts provided with my ReSound custom silicone molds stay in place is that the vent holes are not a straight-through bore but rather bend a bit to follow the outer shape of the mold. Thus, the springy insert has to bend a bit itself on insertion and I would presume that the spring-loaded bend in the insert creates tension against the side of the mold bore hole and keeps the plugs from moving. I’m wearing an almost totally occluded insert - it seems the more solid the insert, the greater the springiness. It’s my vague recollection from trying the more open inserts that the less material there is in the insert because it’s more hollow, the less “springiness” and lesser tension generated against the side of the bore hole. But I’d say the key ingredient is that if you’re bore holes don’t curve a bit and the bore hole is almost straight through, I’d bet the Select-a-Vents are going to be more prone to migrate in the bore holes and perhaps fall out.
I’ve been wearing my molds with the vent plugs about 1.5 years and usually never see more than a few mm migration in a month before I take everything apart to clean the mold in a mild dish detergent solution. As I clean the molds each day with a Jodi-Vac, if I see any migration, I can just push the plug back in place with the blunt tip of the Jodi-Vac needle but I can’t remember the last time I had to do that. Since I wear an almost totally occluded mold and my molds fit my ear canal pretty tightly, there is perhaps a bit of plus or minus air pressure that might tend to move the inserts as I insert or remove the molds from my ear canal (maybe the +'s and -'s balance out?! 
- but seriously, my inserts really don’t move - perhaps also some ear wax gets into the bore lining and helps as “glue,” too?!).
How can one’s mold have a curved “bore” hole. I think in the ReSound instance, neither the receiver cavity nor the vent cavity are “drilled out” but rather the mold is created by 3-D printing.