I can't find anything online or anywhere else offering articles on the possibility that secondary uterine inertia could have a genetic link. I'm just curious as to what you all think about this, could it be or no? I've found it suggested that primary UI may have a genetic factor, but not secondary. Thanks.

I think a lot of uterine inertia has to do with the "hover" factor. The owner is hovering over the bitch while she is whelping. The bitch will be experiencing all kinds of confusing things especially if it's her first litter. She needs quiet time to relax and to be disturbed as little as possible. No matter how good the relationship between owner and bitch, your presence causes her to body to release adrenaline. Adrenaline negates the effects of oxytocin. Oxytocin is what causes contractions. I try to stay in an adjoining room while my bitch is in labor, close enough to hear what's going on but far enough away to let the bitch think she's by herself. If you can't bear not to watch, try a web cam.

O.P. What are you considering secondary uterine inertia to be? I am familiar with primary uterine inertia, had it in my last co-owned litter, and ended up doing a c-section.
I think Nancy is on to something. I hadn't thought about adrenaline as a factor in blocking the release of oxytocin, but this makes a lot of sense. I have always felt if we take all the natural instincts out of our bitches by controlling every move she makes in labor, some will just give up, physically, and mentally.
My mentor in Labradors told me the bitches that historically have the least amount of problems whelping are the ones who are "kennel dogs" and are left alone to whelp their litters. I could never bring myself to do that, but will try to back off a bit in the future.

From what I've researched, primary is labor/contractions not starting at all and secondary is the labor/contractions stopping after 1st pup is born. Primary is basically caused by a lack of hormone to induce it, secondary is caused from exhaustion.

I had a bitch have secondary with her first litter...after her first pup. Subsequent 2 litters were fine and she actually went on to be a fantastic whelper. Her daughters that I kept never had issues either.

Has your girl had this issue more than once?

I think you just hit some interesting questions especially does it occur with all litters. Also there are other factors. Take into account the litter itself. For example large puppy sizes, litter size, how much weight was gained through each pregnancy and alot more.

I've fortunately never seen it happen bad enough to require surgical intervention.