I have been having a terrible time with shooting this year. I feel like my accuracy is off, and when I do hit a bird, nothing dies solidly. I have had to ring more necks this year than ever before. I have been using Herters this year as well. I wonder if that is part of the problem?
It is highly unlikely to have anything to do with your shells unless you are using small shot (7.5's or smaller) and you are shooting birds out farther than 35 yards.
Lots of misconceptions here. First off, shot hardness is important only for the way it affects patterns at long distances. If your shot is soft you will get more setback deformation of the rear-most pellets in the shell, and deformation produces flyers. However, not every flyer leaves the pattern. Some move from the left to the right side, or from the bottom to the top, but remain in the pattern. And it is only the rear-most pellets that get deformed.
At anything out to 30 - 35 yards, you will lose very few pellets as "flyers" caused by setback deformation. Not many people shoot pheasants beyond 35 yards.
Soft shot penetrates the birds nearly as well as hard shot. Take away: shot hardness is not your problem.
7.5's, whether hard or plated or blessed by a bishop, will not penetrate a pheasant's feathers well, and especially not at 30 yards. Sure, guiding I have seen guys with 28-gauges loaded with 7.5's just slaughter pheasant after pheasant, but they were expert shots capable of "head-shooting," and they were passing on any shot over about 25 yards.
6's penetrate pheasants okay and I have killed a jillion of them with 6's, some in very long shots. HOWEVER, I try to avoid 6's any more because too many of them get left in the meat and then bitten by someone's teeth at dinner. 5's are a much better choice in this regard because nearly every #5 will go completely through a pheasant. Some #5's will be trapped by the elastic skin on the far side of the bird, but those fall away during skinning. Very few get left in the meat.
However, there is ZERO point in shooting pellets bigger than #5's, because, as noted, nearly every #5 pellet is going clean through the bird already. Going to a larger pellet gains nothing, energy-wise per pellet, and reduces your pellet-count.
If you are not a good wing shot, use very open chokes and #6's, and limit your shots to 30 yards. If you are good, use #5's, tighter chokes and the sky is the limit.
So what is your problem? Well, it is one of three things:
1. Your gun does not fit you;
2. You are simply not a good shot. Nothing to be ashamed of, very few people who do not shoot at least 20K sporting clays targets every year are good wing shots, and not all of the ones who do are.
3. I miss some birds every season to pure bad luck -- I was pivoting into a shot and stepped in a hole or branch hidden by the grass. I miss some because the lucky bird just happened to zig left or right just as I was shooting.
Some lessons will help with #2, and seeing a good gun-fitter will help with #1. Ben Hustwaite is right -- avoid any instructor who would have you shoot a pattern plate. Instead find one who can see where you are missing when you shoot clays. If you are consistently high or low, your comb may need to be lowered or raised. If you are missing left or right, you are probably doing something wrong with your eyes just before and when you pull the trigger.
But my first thought upon reading your post was, your gunfit needs can change over time! I realized mine had last month when my trusty old 20-ga. O/U was not working well during my annual SD pheasant hunt. Cleaning the birds revealed that I was consistently fringing them with the top of my pattern, IOW, shooting low (not just on rising birds, on crossers, too). I brought the gun home and raised the comb by bending the stock up about 1/2" at the heel, and now it is spot on again. I don't know why my fit needs changed, I just know they did, and when that happens, you need to change the gun -- you cannot change your body.
P.S. Don't try to bend your stock yourself, hire a professional. There are many good ones around the country. The photo below is not of my 20-ga, it is a friend's O/U getting a bunch of cast removed.