1.) Make sure the slide stop pin is lubed, just a drop of light oil. SS's are always tight on ANY
new 1911 and should be checked for lube prior to using the gun. Also check for wear and/or galling as the round count goes up. This will indicate miss-alignment of pin to barrel link.
2.) Lay the two mags side-to-side and inspect the slidestop interface for differences.
You should find your problem with one of the two suggestions above.