I have been reloading and trying different loads for my 308. Shooting for accuracy using different bullets, powders and charges. It seems the most accurate loads are newly loaded cases. These outperform my previously loaded and fired cases. Can any fellow reloaders explain what might be causing this or suggest what I can do to remedy this?
Thanks in advance.

Welcome to the forum!
Give us a little more detail.
What kind of rifle? AR? Bolt action?
What are you doing with sizing? FL? PFL? Neck sizing?
How are you setting up your sizing die?
What kind of accuracy are you getting with the different loads and what are you trying to achieve accuracy-wise?

your load info would be a good place to start as well. A load that works for me is 168gr sierra GK H414 in a Douglas barrel 1:10 twist.

Hello and thanks

I am reloading for a 308 bolt gun, Rem 700 in a Bell and Carlson medalist stock. I have only been necksizing the cases using a Redding neck die. I set it up by running it down to the shell holder then backing it off approximately 1/8 of a turn. I have used IMR 4064 starting at 40.5 grains and worked up from there at .5 intervals to 42.5 grains. Also used Reloader 15 starting at 40.5 grains and worked up to 41.5. Bullets used are Sierra 168 grain match kings, Hornady amax 168 grain and Nosler ballistic tip 165 grain. Cases reloaded include Remington and Winchester new and once fired. Ive measured the shoulder length of fired and new cases to determine how much head space my chamber is allowing the shoulder to move during firing and have determined that it is only moving a minimal amount compared to new case dimensions. The head space dimension must be minimum as it does not allow much movement. I used the RCBS head space gauge for these measurements. Ive tried seating the mentioned bullets at different lengths from 2.775 to 2.825. No matter what I do the reloads in new cases are shooting groups ranging from .25 to .50 inches at 100 yds. with various recipes and the once, twice, three times fired cases using same recipes produce groups from about 1 inch to 3 inches depending on what bullet is used. The Noslers consistently produce 2 to 3 inch groups in all cases fireformed or not, Hornady amax and Sierra MKs produce 1 to 2 inch groups in fire formed cases. I thought reloaded, fire formed cases would shoot better groups than new cases loaded with same recipe. Seems this is not so in my particular experiences or im doing something wrong during my reloading procedures of fire formed cases.

Maybe the chamber isn't quite concentric with the bore?

I would say you need to readjust your neck sizing die so that you are only sizing the neck of the case, having it set so deep in the press you may be pushing the shoulder back. I seldom use a neck sizing die on a beltless case as I've almost never seen the need. In my experience if the dies are set correctly the loads shoot the same whether the brass is new or previously fired.

What Ld said. Have you tried FL resizing the fired brass and reloading it?

This is a new one on me; usually it is the other way around. Has to be something about your die or the way you are using it.

Have you indicated your loaded ammo for bullet runout? Your loading procedures need to produce straight ammo with minimum bullet runout.

Also if your chamber runs out (significantly) axially from the bore axis this can make for a situation in which the loose fitting new or fl sized rounds can be allowed to align better than the fire formed stuff. I know this sounds counterintuitive but I do think this happens sometimes.

Another thought .... did your best accuracy happen when the gun was new and Your brass was unfired? And now the once fired stuff isn't doing so well? Might just be coincidence. It's possible the Stock bedding has gone south. I have seen this happen in under 200 rounds, and with a Remington .308 too.

Anyway just some thoughts.

I would just start over! clean the bore really good make sure there is little or no copper fouling.
If all you brass is on the 4x firing.
I would full length resize trim to length to square up the mouth of the case. aneil the case necks. the way you stated your neck sizing is you are setting the shoulder back. after the first firing I would just neck size set the die to size only about 2/3 of the neck chamber a few if there is a lot of resistance with the bolt turn the die in 1/8 turn at a time until you feel a slight resistance when the bolt is closed. make sure to chanfer the inside of the necks and use a lube inside when sizing.
when seating a bullet use smooth slow deliberate strokes.
always measure to the ojive of the bullet for consistent seating depths.
shoot a few shots to foul the bore before shooting groups.

+1 on what Ld and Hi said above.