On a new build, if the hammer string is installed correctly and using a factory upper (that should have proper headspacing) what else would cause light primer strikes?

Have you pulled the BCG?

Looked at the firing pin? The firing pin channel thru the BCG?

With no spring to fight in an AR, 'congestion' increases resistance, and the inertia imparted by the strike of the hammer is lessened.

Combine that with hard primers (eg Military) and sometimes you get click instead of bang.

Are the legs of the hammer spring on top of the trigger pin where they belong?

I replaced the buffer and it is working correctly now.