I had the same issue, but Ruger wanted me to send the gun back as opposed to swapping out the one mag that was giving me trouble.
I had an extended conversation with the tech at Ruger. Basically this is a tolerances issue. A number of mags got out that are within manufacturing spec, but due to minute differences in manufacturing tolerances (which is inevitable) are failing to reliably engage the magazine disconnect because the magazine disconnect is recessed too much. I assume they are going to install some type of bushing that will push the disconnect out so that it is reliably actuated by all the magazines out there.
Even though this issue likely could be resolved by swapping out the bad magazine, I guess on balance its better to send back the whole gun and have this modification completed so that going forward, the gun will work with all the magazines available for it. It is damn frustrating to have to send back a brand new gun that I have been in possession of for less than a week for a problem that could have been easily avoided by just leaving out an extraneous, useless, and nearly universally loathed feature (the magazine disconnect).
This is the number one reason I would never buy a strictly defensive firearm with a magazine disconnect safety, and though I am a huge fan of Ruger (I have 10 of them) I will never give up my Glock for serious social work.
I'm not going to be fully satisfied unless they throw in an extra mag or two when the gun comes back.