Hours of research and more parts swapping later...success! My buffer tube has a notch cut out for the buffer retainer. I unscrewed the buffer tube one rotation, so it barely holds the retainer, and what do you know...the bolt will lock back now. The one rotation was just enough room to let the bolt travel the 1/8ish needed for the bolt catch to engage! So now I will be looking for a buffer tube with a slightly longer lip to hold the retainer better. Thanks again for the help people.