Telescoping stock spring should be close to 10.5" long.
Carbine buffer should be 3.25" long.  
The telescoping butt stock carbine receiver void depth from the end of the tube lip, to the bottom of the channel milled channel, should be 7".

If these are good, then assembly the rifle without the charging handle, use the door detent on the side of the carrier to start the carrier back/unlock the bolt from the barrel extensions, then get in front of the bolt with your finger to shove the B/C all the way back until the buffer bump touches the back of the tube.

As for my bet, you either have a knock off receiver extension that is milled 6 3/4", or a rifle spring (11.75") that is coil binding up before the buffer bumper can touch the back of the tube in the void.

As for weeding out the charging handle, then just put the rifle together without it to check.  Us the detent in the side of the carrier start the B/C back, then get in front of the bolt with your finger to shove it all the way back to see where the bolt stops against the back of the ejection port window.


To add, the lower receiver parts have nothing to do with the upper receiver!!!!!  
The receiver extension void depth dictates what buffer and spring  are used.  

Hence if you have a fixed stock on the lower receiver, then the fixed stock receiver extension void is deeper, and requires a rifle spring and buffer.  Simply the main purpose of the these three is to stop the face of the bolt in the correct position. So if you use a carbine buffer and spring in a fixed stock tube, the back of the carrier key will crash into front of the threaded section for the tube, and crack the lower receiver. The other way around with a full rifle buffer and rifle spring, the bolt will not retract back far enough instead.  Again, face of bolt should stop 1/8" to 1/4" in front of the back of ejection port window edge with a full charging handle pull.

Lastly here, the same length receiver extension tube is used for both the A-1 and A-2 butt stocks.   The difference, the A-2 butt stock used an end of tube spacer of 3/4"  to fill the void between the end of the tube and the inside of the butt stock channel for the tube, and the butt stock screw is longer for the A-2.  I bring this up, since if you are using a A-1 butt stock, then you need to use the 3/4" shorter A-1 butt stock bolt, or the butt stock bolt will be protruding into the receiver extension void to stop the buffer fro being able to reach the bottom of the tube instead.