If the rifle cycles fine and only issue is that the trigger doesn't reset, this eliminates upper issues, magazine issues, and ammo issues.
My guess is that the trigger/disconnector have been assembled incorrectly. In particular, I'd bet a small amount (these are tough times) that the disconnector spring is installed upside down.
Here is the test:
First, with rifle unloaded, separate the upper and lower; put the upper aside.
Second, works best if you can chuck the lower using a "Flipper" or perhaps a cleaning holder that grips the butt stock.
Third, be sure that, in doing the following, you do not allow the hammer to strike the rear of the magazine well, which it will damage. Either use a hammer strike block or your fingers to capture the hammer fall.
Test: With finger off the trigger and safety off, cock the hammer. Based on your report, it should cock fine. Now, pull trigger (again: capture and cushion hammer strike); hammer should fall. Do not release trigger. With your other hand, and while still holding the trigger back, recock the hammer; it should be caught by the disconnector. Now (again, taking care not to let the hammer fall and strike the magwell), release the trigger slowly and see if the disconnector releases the hammer to the sear face. If it does not, you have a hammer/disconnector problem. As I said, most likely an assembly error and most likely disconnector spring installed upside down.
