I just got a complete upper, t put on a lower that i put together out of parts, and pieces. everthing went together well. when i took it out to test fire it. the fist round fired great. when i put a couple of rounds in the mag, the round fired, but when i went to fire the second round. nothing happened. I cycled the round out of the chamber, and it was unfired. so i cycled a new round in, and fired it. same thing the second time. so i seperated the upper, and lower. if you push the hammer all the way down with the trigger in the full back pos the disconect catches the back of the hammer. which i think is normal. but if you release the trigger very slowly. the hammer falls. if you release the trigger fast, it catches like it is ready to fire. so there is my delema, any suggestions would be appriciated. thanx

First question is what make lower did you assemble your rifle from? Provided that you used a NEW (unmodified) trigger group the only reason I see that happening is that the trigger pin holes are out of spec or your disconnector is binding somehow. Is there a spring under the disconnector? Does the dissconector move when you pull on it? (This will take a little force because of the spring)

Your problem is with the semi auto disconnector and or hammer. You used inferior quality parts kits parts. You need some quality parts from Bushmaster or other name brand manuf. not gun show parts.

I agree,

I would gut the gun & find out if the parts are all the same manufacturer parts.  Compare with bushmaster, RRA,  etc.  Whatever the Reciever manuf is.  See if they are the same as,  or diff from the manufacturer of the reciever.  If all else fails you should start from scratch & get a lower parts kit...  Bummer

Prosise

It is also possible that he installed the disconnector spring upside down.  As stated earlier, make sure that your disconnector will pivot.  What it sounds like to me is that the disconnector is hanging.  Try removing the trigger assembly and checking for burrs.  Lube the pin holes in the trigger and disconnector.  Make sure all your springs are properly oriented and reassemble.  

How do you know what he used? Just wondering.
I would go with trigger spring installed upside down or backwards .
GG

Because typically guys don't put kit guns together from Quality parts. They buy stuff online or from gun shows. That's how I know.

Also Bushmaster, and Colt and Olympic, and DPMS parts do not have this problem right off the bat when they are new.

And yes it could be a binding or upside down or weak disconnector spring.