Same here, mostly. Once in an while you'll get a spring that even when compressed is SLIGHTLY too tall to engage the nut in the stock holder. In that event, you can grind some with a dremel on the spring, or replace the spring, or replace the nut with the other part.

Just be aware that the 6575 nut has smaller 'wings' in the assembly that will usually shear off before the nut strips, and there's darn near no way to get it out at that point. At least with the stock parts, the nut will strip out if you over torque/over compress the spring and allow you to dissasemble the diff and replace the broken parts. NOT always so with the 6575 nut! Sometimes when the wings shear, it's a total loss. Been there, done that.

Usually, though, the stock nut/holder not engaging is caused by either not seating the nut completely in the holder or not pre-compressing the spring sufficiently, or both.