Before you light your hair on fire and go screaming like a banshee back to the dealer, or Remington, I am thinking you should at least try a few things.
You say you are new to shotguns. OK. Most folks when they first get a new pump treat it like it is Grandpaw's railroad watch. It ain't. If you don't rack the slide with authority, you will get results like you mention. Now, I don't know if that is the case or not, but you can find out by racking the slide like you are trying to break the shotgun. You won't. You can't.
Watch someone shoot an auto loader. Does that bolt gently creep back? No! It slams back like a locomotive. That is the action you need to emulate with the pump.
Next, have you read and followed the owners manual? Did you heed the part where it tells you to thoroughly clean the gun before you shoot it? Remington coats these guns with a rust preventive that is brown, tacky, and generally not conducive to good operation. Clean it thoroughly, and lubricate it well.
Give it another try. The 870 just loves to be shot. The more you shoot it the better it gets. At least that is the experience I have had with all three of mine. I have an older Wingmaster in 20 gage that is just a thing of beauty. I also have 2 Express 12 gages that were purchased within the last 3 years and have yet to give me anything but admirable service.
