I got a new Remington 870 on Friday.  Since I had to work that day I couldn't go to the range to test it.  

I loaded the six round magazine with safety trainer dummy rounds to do a feeding test.  It would not feed the last round out of the magazine.  I had to remove the magazine cap and run a cleaning rod down the magazine tube and tap the magazine follower to get it to push the round against the tabs that hold it in place until the loading gate can pick it up.

I took the shotgun to the range today and this time the second round would not feed properly.  I placed the shotgun on SAFE and slammed the butt against a table.  This moved the round against the tabs and allowed me to manually unload that round.  I then worked the action and the next round fed and chambered like it is supposed to do.  The round I manually unloaded, I re-loaded into the magazine.  I then fired four rounds in rapid succession and had no problems.

What could be causing this problem ?  Could I have gotten a shotgun with a defective magazine spring ?

It's possible the shorter spring for a sporting model got put in but I'd check the stamped follower for burrs or deformities. The new Remlin guns have their QC issues.

Remove the mag tube cap, spring and follower. Sounds like there may be debris (from the Styrofoam carton?) or something else hanging up the follower. Check the cheap plastic follower for cracks or mold marks while you have that part out too.

Thank you.

The factory Remington followers are garbage.  Replacing that would be my first step.

Could be the dummy rounds.  I have some of the orange plastic ones that will no longer feed correctly from being "chewed up" by years of reloading practice.  
I would suggest cleaning and inspecting the magazine tube and follower as stated above then try it with some live rounds at the range.

Swap for a GGG follower with red center.  Just beware you may have to remove the tabs at the distant end of the tube to install.  Its a 2 min job with a drill or dremel and file.  Makes cleaning the mag tube easier. There is a sticky guide here on itThat plastic orange followet is cheaper than. 20 oz soda lid and should be metal for what we pay them.  You can also get an original metal one used if you want.  I had the same problem with the last 2 870s I bought.  Try the things others said here first

I had an orange plastic one that caused feeding problems on a new gun. I buffed it with a little steel wool and chamfered the edges slightly, cleaned the mag tube and then sprayed everything with some Silicone lube. Not a problem with it since then. This is a fun gun for the range. Anything you may bet your life on should get an upgraded mag follower.

OP did you buy the factory tactical? Thats the same type I had issues with
 

If you want to make it better for HD You can for about 100 more.  XS big dot sight, new follower and safety or sling plate.  Makes an awesome set up