Could be a weak spring, or oil or dirt in the firing pin cavity on the slide. If you have an ultrasonic cleaner you might stick the slide in it with some solvent and give it a few minutes of action to see if that frees up any crud in there.
This^^^^^ If it has a hard strike on all but one or two, you need to see if you have some junk floating around the firing pin. Springs are consistent for the most part. If you have a bad one, it will be consistently bad. Lube could be an issue.
I see some posts in other forums that say the primer may be too deep. I guess that could happen if your using some hand primer press and got the wrong pin in, but my progressive press cannot press one too deep. Its always flush the the base of the brass.