My buddy had to just slightly file his poly mag splines, cross mags are fine, Gi metal mags are fine, some metal magazines required a slight squeeze in a vise along the back spline. Now they all drop free and function fine in his Gen 3, as well after the slight fitting they still work fine in his WK.
His FX9 seemed to have problems when his poly Glock mags when loaded up as well, I am not a fan of non metal magazines.