With “rookie” in your title, I am thinking that you short-stroked when you first attempted to chamber a round. IOW, you did not pull the charging handle ALL the way to the rear, before letting it go. The charging handle may have reached a point at which it was in a bind, providing a false indication that it was all the way to the rear.
To be clear, short-stroking can happen to experienced shooters, too, which is why it is a best practice to verify that a round has been chambered AFTER dropping the bolt, or running the charging handle.
Plus, “rookie” is not a shameful title. We all start as rookies, when learning a new weapon system, and I know that I can revert to rookie status, with the AR15/M4 system, if I do not train regularly, as several aspects of the AR15/M4 system are NOT intuitive, to me. I first learned how to use an AR15 at 40 years of age, whereas I learned pump shotguns in my teens, and several handgun systems in my early twenties.
1. Starting with rifle empty, hold the loaded magazine in support hand, and look to see whether the top-most cartridge is to the right or left side of the mag, then, Proceed to Step 2.
2, Insert magazine, until it clicks into place. Pull on mag, to provide an indication that it is properly retained.
3. Drop bolt, or, run the charging handle, depending upon how you started the process*.
4. Then, one option is to remove the mag, and see if the top-most cartridge is now on the opposite side of the mag. This will indicate that the original top-most cartridge has chambered.
4a. An alternative to #4, above, is the mis-named “press-check,” to verify that a round is chambered, but then, one must be SURE to close the bolt COMPLETELY. (“Press-check” does make literal sense when applied to the old-school method of verifying that a 1911 pistol is in a properly-chambered state**.)
5. If one has performed #4, rather than #4a, re-insert mag, and when it clicks into place, pull downward, to verify proper seating. The Forward-Assist can be helpful, to ensure the bolt is closed.
Either 4 or 4a are valid procedures, during administrative handling. One nice thing about #4 is that it can work in total darkness, with only the support hand being moved from its place on the weapon.
*If the weapon is for serious purposes, it is best to be familiar with both, as a defensive incident, or intense competition, could result in one finding the bolt carrier group in either position, at reload time.
**The old-school 1911 Press-Check was so-named because one placed the support-hand thumb inside the forward inside surface of the trigger guard, and the index or middle finger of the support hand on the forward portion of the recoil spring retaining cap. It is no longer considered a best practice to do this, as there are FAR SAFER ways to verify the condition of the chamber. The thumb being inside the trigger guard is no longer consider properly safe, and, unless the pistol has an extended barrel, placing any body part on the far end of the recoil spring assembly area places it just too darn close to the path of the bullet.
