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I have had the same problem a couple of different times.  Make sure that your gas block is tight against your end cap and it tightened down.  Mine was loose allowing gas to escape-not allowing the bolt to be pushed far enough back to strip a round off the mag.

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as far as ammo,  its all been pmc bronze. i know its not the best stuff, but its not steel cased and there should be no reason why my ar wont eat it right? i'll check the gas block when i get off work. how can i tell if its loose? i just read something about using high temp wheel bearing grease on the bolt? i've never used anything but regular gun oil? should i be using grease?

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@foursixty      Great info and i really appreciate you talking english about it lol I will check the gas block when i get home from work. as far as the 5.56 ammo - my rifle does say .223/5.56 so the hot stuff should be g2g right? and one more thing - when you said "soak" the bolt carrier in lube do you literally mean let it soak in it? sorry i'm way new to the AR world. I do clean it after every time i shoot, but i would definately not say that i over lube this thing. apparently i should be.

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never over lube you gun just a waste of gun oil use it on the sleds that touch the buffer tube/ upper reciever on the bcg, check that gas block, and buy some quality ammo like remington fmj 55gr or try federal XM193f 55gr.

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Your quite welcome.  It makes me feel good to help a fellow enthusiast.  I am one of those people that doesn't know everything,––far from it, but I will be straight up with you and tell you what I know.

As far as using 5.56 you should be good to go.  Very few of the DPMS models are .223 only anymore.  And the majority of the ones that are .223 only are the hunting/varmint oriented rifles.  If you look at the underside of the barrel it should say something like " DPMS 1-9 5.56".  As long as it says 5.56 on the barrel you are good.

And the "soak" was just my lamens attempt to get you to understand wet.  I know that sounds dumb but if your using a spray lube just hose that thing down.  I mean dripping wet bolt and carrier.  Then just wipe off the excess gently and that's what I consider "wet".  Everybody has their own methods, and this is probably "too" much.  But in my experience there is no such thing as too much.  AR's love the oil.  I think you said you had 400rds or so down the pipe??  After you get some more it should smooth up and cycle even easier/better.  Keep in mind the only method I can give you is my method, and that by no means is the only method.  It's just what has worked well for me.  

You will not hurt it using too much lube.  If anything you might get a spray in the face when it cycles or slick hands.  My guns are wet enough after I clean them that oil seeps out of the safety selector, trigger pins, and upper and lower joint.  Don't hurt a thing.  Just wipe it down.

ETA:  Also, as far as ammunition goes:
I have had real good luck with the AR Amercian Eagle both in 5.56 and .223 and the Federal bulk packs.  You can find these at Walmart if your local wally world sells ammo.  They will usually be the same price or even cheaper than the PMC.  And the American Eagle and Federal Bulk packs are pretty much the same bullet.  I can tell absolutely no difference.  These aren't precision perfect MOA capable rounds, but in my experience they have proven reliable, affordable, and consistent performers.

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Will definately break her down and try all this. does the federal and american eagle .223 stuff run hotter than the pmc bronze?

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never over lube you gun just a waste of gun oil use it on the sleds that touch the buffer tube/ upper reciever on the bcg, check that gas block, and buy some quality ammo like remington fmj 55gr or try federal XM193f 55gr.

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i bought the pmc bronze for the price to be honest. anything better for close to the same price?

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Trying to pinpoint little areas to apply lube will end in your gun not being properly oiled.

Oil isn't that expensive.

I have fixed a lot of other peoples' firearms at the range/matches/quals by adding copious amounts of lube. They usually tell me that they did the exact lubing procedure that was in the manual or that their Drill Sgt told them to do - that's all a load of crap. These, and most guns, like to run wet.

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well i checked the gas block and key and they both seem tight. and also it says dpms 5.56 1 9 on the barrell so the hot stuff should be g2g right?

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Yep.  

Barrel is what you go by every time when in doubt.  

If the block and key seem tight we're doing good.  Means we're eliminating variables.  
I would try to lube it heavier and try a different ammo.  Although the PMC works great in lots of guns you may have gotten a weaker batch then usual.  Like I say, even the PMC I run in mine without problems is noticeably weaker...VERY.

Basically any gun is a simple machine.  Some more complicated than others.  But they are simple machines nonetheless.  Before you look any deeper than necessary, eliminate the ammo as a variable.  Who knows, after you get her running and get a few more rounds through it you may very well be able to come back and burn up that PMC without a problem.  Stuff like this happens.