I'm baffled. The only thing I can do is tell you what has worked for me. When do you remove the eggs from the turner? I assume that at that time, you do not turn them by hand. What is your rooster?
An other thought: When I'm collecting hatching eggs, especially if collecting them before the flock gets out to free range, I provide multivitamins for about 2 weeks before I start to collect. I also give my flock sprouts during the winter months, and ferment their feed year round.
I'm guessing that your Ambers and Sex links lay a pretty large egg. I've found that the large eggs are problematic when it comes to hatching.
Could you get some barnyard mix fertile eggs from a local farm, and try those to see if the problem is in your flock, vs. your incubator?
Here's a link to a compilation of mostly research based hatching tips. Since I last hatched in an incubator, this treasure trove has been revamped. I find this new revamp to be much busier, and more difficult to sift through. BUT, if I was going to fire up my incubator again, I would take several hours to sift through this info. Sally Sunshine is the most knowledgeable person re: incubation that I know.
https://www.backyardchickens.com/articles/incubating-and-hatching-chicken-eggs.64195/
Other questions for you: Does your bator have a fan? If so, hatching temp should be 99.5 at egg surface. However, I prefer to use 100*F, and have wonderful results at that temp. If your bator has no fan, then the temp should be 102*F at egg surface. I use 101 - 101.5* for still air. I don't like to go to 102*, b/c that is just a tad off from egg killing 103* temp.
Here's an other article that you might be able to glean from:
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/2902/2902-1090/2902-1090_pdf.pdf
And an other one:
http://extension.msstate.edu/content/pipped-eggs-do-not-hatch
I strongly advise against using vents to control humidity. The chicks greatest need for O2 is during the final days of their growth.
Could be that you are running too wet through day 18. I'd suggest that you drop back to 30% through first 17 days, but use the air cell chart to guide you. IMO, better to have air cells too big than too small. This site has some good info. I've lifted a paragraph from it to support my "egg too wet during incubation" stance.
https://poultrykeeper.com/incubating-and-hatching-eggs/incubation-humidity/
If the incubation humidity is too high
When the humidity is too high, less moisture is lost from the egg and as a result, the air sac is too small in size, the membrane is less brittle and much more rubbery. Think of the air sack like a small balloon: once inflated, it’s easy to pop but while deflated, it remains thick and rubbery.
A chick trying to break through this membrane can expend too much energy, or can suffocate from a lack of oxygen before it manages to break through.
In addition to the difficulties a chick will have with internal pipping, if the egg hasn’t lost enough moisture, the chick cannot rotate inside the shell and becomes stuck. The fluid around it dries out once the chick has pipped making life even more difficult. Unable to break free from the shell, it will eventually die.
The reality though is that the porosity of eggshells vary (becoming more porous later on in the year) and vary between different breeds and with different thickness of shell, so the correct humidity is whatever achieves the correct moisture or weight loss from the egg which is generally somewhere between 11 and 13% when the air sack occupies approximately a third of the egg at the point of internal pipping.
The best way to do this is either by recording the weight of the eggs using some accurate scales, (see my article on the weight loss method for incubation) or more simply by candling your eggs and comparing the size of the air sac to a diagram that shows the ideal size on different days of the incubation period.
If the air sac is too large, the eggs are losing too much moisture and you need to increase incubation humidity and conversely if the air sac is too small, decrease humidity so the eggs can lose moisture faster.
