I misunderstood the opening post to mean you had already tried the troubleshooting steps and so failing those I usually fix a 750/650 seating depth issue with the adjustable depth stop.
If we are backing up a few steps, the low hanging fruit is a combination of making sure the shell plate play isn't too loose, and also making sure the primer cup ram on the 750 style primer mechanism has not slipped.
I'll see if I can find some videos or snapshots.
And the ram in the cup can go too low as well as too high. It is tricky to adjust, but you will get the idea here, which I am bringing up the opposite of this procedure which drops the ram instead of raising it. By that same lock adjustment screw, you can raise it but it is tricky.
Once you go through all of those, then you get that aftermarket part that gives you control of where the stage stops on the prime stroke and never look back.