Unless there is some subclinical disease in your flock that affects hatchability, it sounds like you have been doing everything you need to do to. The best answer in that case is to import hatching eggs or chicks from another source. LF Wheatens and Blue Wheatens all go back to one original source as far as I know, that of the late Wayne Meredith. So the chances are pretty good you would not be doing that drastic of an outcross. Substandard fertility and/or hatchability, along with reduced production, are some of the first signs of inbreeding depression. To avoid that for long periods of time a person needs to keep about three different lines going in order to maintain a closed flock.
They likely came from the same original source is some food for thought…
Just for clarification about keeping 3 lines separate, you mean acquiring birds from 3 different breeders and maintaining them separately, and crossing them in as needed or setting up 3 pens from 3 different breeders and rotate the males from the different pens?
My birds have been healthy and lay pretty good despite most of them being 3+yrs old. I’m definitely not complaining there. And they are super peaceful with each other.
I'll simply say that I maintained a closed-flock of WBS for 15yrs without ever experiencing that problem. Then the HPAI hit and all my birds were put down. I don't wanna go into detail here but I was able to salvage my bloodline so it wasn't lost. I'm about to set my first eggs from a Quad. They're not the quality I had before all my birds were put down. I'd say 2-3yrs behind where I was but it's a start.
My Am's have always hate the worst of my hatch rates but that's primarily been because of porous eggs that develop bloodrings. Don't recall any real problem with quitters and definitely don't recall any kind of positioning problem. I've actually not heard of that before with any Wheatens but I haven't been keeping up with what others are doing near as much as I used to. Pretty much not at all.
Have you been able to identify whether the quitters are coming from a particular hen? I think that'd be one of the first things I'd look into.
Sorry I can't be of more help.
I don’t remember the porous egg issue when I had your stock. I definitely had issues with them not developing. I didn’t have that issue with my current stock at first. After posting this, I went down a rabbit hole on feed and nutrition may be a factor with the positioning issues. The supplement I started adding a month ago should close any gaps. Hopefully this last batch I set will show some improvement. So maybe it was 2 separate issues?
I’m having fertility issues in all pens so it can’t be just one female family or single hen. My last batch I candled was even worse so I switched all my males out and put my outcross line over the hens. They've been separate so far other then the original cross with some of my hens over the hens. If fertility jumps, I guess I’ll have my answer.