AR 556 not holding mag
I bought my first AR today. So I'm pretty new at this. I work in a gun store but don't have hands on much as I am in accounting. I brought my firearm home and started getting familiar with it. When I inserted the mag, the "catch" wasn't happening. I can slide the mag out without pressing the release. After reading some forum posts, I separated the upper and lower and tried again. It worked. But when we put the upper back down, same thing, no catch. Anyone have any suggestions before I have to call ruger? Thanks in advance!