While you could have a bent shell latch, my guess is that the greater probability is that you failed to push the last shell you loaded far enough up into the mag tube to engage the shell latch (with the 870 you should actually have the tip of the thumb slightly inside the tube before you release the shell0 Since you say you loaded "about" 5 shells into the magazine, I am unsure if this shotgun has an added magazine extension or not. If so, sometimes the spring provided with the extension is longer than necessary and begins to give a lot of resistance after about 5 rounds have been loaded, thus making it even easier to fail to push a shell far enough into the tube. If there is no extension, then the standard tube will only hold 4 shells, of course, so a 5th is not going to make it all the way in. (These were 2 3/4" shells, correct?)

If you do, in fact, have shell latch issues, I would either replace the offending latch with a new one, (which should be correctly staked into place) or send the gun to Remington for an checkout and, if needed, an overhaul. Those latches are critical to correct operation of the shotgun, and if this is to be a serious HD gun, I would not take a chance with home gunsmithing.