Ok this past weekend I bought a FN contract upper from a tommys uppers and a rock river lower.  I'm having short stroking problems where the action cycles but only sometimes kicks the empty out and never rechambers a new round.  I just sprayed a bunch of cleaner down the gas tube and it came out pretty clean.  Tommy assembled this upper for me from parts.  Its got a picatinny rail gas block instead of a front sight post.  If I pull the charging handle back and let it go slowly it will not strip a round out of the mag.  I can help it alone with a slight tap to the forward assist and it rockets forward with a round.  The only thing I can think of besides the bolt being sticky is the gas block not being positioned right.  Id rather not go knocking pins out yet, but will if I have to.  The rifle shoots dime groups at 50 yards with a red dot scope so I'm very happy with the construction, just not the operation.  What should my first point of attack be?

Weak buffer spring?  Burr on bolt catch?

Sounds like a gas problem.Check the bolt gas rings,the carrier key,mis aligned gas block or a gas tube that is not locking up to the key right.You can check the gas rings by inserting the bolt and then holding the whole assembly by the bolt nothing else installed.If the carrier does not fall off you have a tight seal.Check the key for loose ness and the gas tube for straightness or to make sure there is no damage to the end that would prevent a positive seal from being made.I had a tube that was worn a bit on the left and was not getting a good seal on lock up.Shot fine with full power stuff but with lighter loads I was experiencing the bolt not locking back and a new tube solved this.Also you can feel it in the recoil if its light or the carrier  sluggish its a gas problem.

I thought FN upper were for military contract only

The way it was explained to me is that FN rifles are mil contract only.  Rifle parts are not.  The carrier seems to be sticky all the way through the action.  When I stuck a compressed air canister hose down the gas tube to blow it out gas escaped around the gas block.  Not much but some.  Is that supposed to be super duper dolfins butt hole air tight or is a little leakage acceptable?

Make sure the carrier key isn't loose, next rings, next make sure the gas tube is straight within the receiver and isn't draggin on the carrier key which could make it leak also and not feed properly.  Your gas blocks gas port may not be lining up properly with the barrels gas port.  If everything else checks out you may need to remove the gas block and mic how far the gas port is from the end of the gas block and then the barrels gas port from the machined notch in the barrel (where the gas block rests).  I know from experience that the PRI gas block on a DPMS barrel on my wifes M4gery had to be moved .071" forward to get enough gas to operate properly, funny how two different companies equipment has tolerances that far apart.  Before moving it, it had only been getting about 5% of the gas it was supposed to and wouldn't even move the bolt far enough rearward for ejection.  Runs like a top now with an H buffer even on weak American Eagle and Natec ammo.  

Edited to add.  You may need to adjust your mag catch if its not picking up the next rounds also, or your bolt catch is out of spec.  Check that gas tube within the reciever, that to me sounds like the culprit of the sticky bolt.

IIRC 20% of the gas goes through the gas tube to the carrier it if the system is good.   There should be VERY LITTLE loss through the gas block.