I recently built a upper, it's a BCA right hand side charging bcg upper assembly with 20" 223 wylde barrel with a 750gas block, it's what the barrel calls for and a rifle length gas tube. All new parts. Barrel nut and hand guard is a ar stoner and a silencerco flash hider. If that stuff matters. The lower is psa lower off of my other ar, thats is a 16" carbine. So when I shot the bcg only goes back about a inch and that's it. Again all new stuff. Except the lower. I have read some stuff online but looking to get more specific to what's happening to mine thanks in advance

You sure the gas block is aligned with the gas port?

You sure you got the gas tube inserted fully in the gas block?
The gas rings on the bolt intact?
Did you happen to install the gas tube upside down?

Yes it has the alignment dimple in the bottom of the barrel. Lines up good. I took the see block off with the tube and blew through it and felt and heard the air come out. The gas tube isn't upside-down. Never thought to check the bolt considering its brand new.

Is there a significant gap around the gas block and barrel?
If the gas block was machined too large ID, you might have too much blow by until it seals with carbon.  
You can bed the gasblock with red loctite to speed up the process.

Ok so I found the the bcg was really hard to pull back to took the bolt out and it was very dry and crusty. So I cleaned and put oil all over and now everything is moving smooth. Next step it to fire it. Will keep you updated  thank you

We assumed you knew to clean and lube.
There is a tacked thread on the subject here somewhere.