I finished my 20' build and I am having some problems with ejection and feeding. I have fired a total of 12 shots through the rifle and it ejects each round, but fails to pickup the next, or it will not hold open the bolt after firing a single round loaded in a mag.

Here is what I have tried so far:
Cleaned bolt - gas rings are in proper position
Used a different bolt assembly - same problem
Removed the gas tube and reinstalled - same problem
Tried different kinds of ammo - same problem

When seated in the upper, the bolt assembly can be retracted with the charging handle past the bolt catch and will lock back if I apply the stop. The gas tube pin is in place.

Parts list:
DPMS Barrel and gas tube
Del-Ton Upper
Used Olympic Bolt

The barrel, gas tube and upper are brand new.  Any help would be appreciated.

We just ran into the same problem this week on some rifles we built. Our problem was the machinist drilled the gas port a little small and it wasn't getting enough gas. Opened them up to .093 and they run great now.
Call me tomorrow and we'll see if we can hook up and check it out.

Since you tried a different bolt/carrier assembly, that narrows it down.

I bet it is either your gas port/block alignment, or that crappy DPMS barrel has a tight chamber.

I had a gun that did the exact same thing. After dicking with it, back it went to DPMS. They concluded the chamber was out of spec (too tight). It caused the spent case to stick. The gun lost so much energy ejecting the spent case, that it didn't have enough left over to kick the carrier far enough back behind the mag.

Problem fixed. The stock spring was too long. I took one from another rifle and it runs fine now.