Not reading #9 in troubleshooting, I built my first AR.
It will not cycle back far enough for the bolt catch to work. (I cannot pull it far enough back to engage it.)
It has a carbine tube, I checked spring length and buffer length. Buffer is 3 1/4” The spring was 12 1/4”, replaced with a new spring, 10 1/2”, still won't work.

My upper functions fine with a friends lower, my lower won't function (engage bolt catch.) with his upper. Bolt wont retract far enough.
Figured that narrows it down to my lower. Basically went through and disassembled and reassembled lower. No go.
In frustration I went down and bought a new tube, spring, and buffer kit. Installed and still won’t work.
Here is a picture without the spring with just the buffer and bolt in the tube, still about 1/8th shy. What did I do wrong?
http://i1138.photobucket.com/albums/n537/wchutt/Stuff/IMG_4331.jpg

Stag lower, armalite parts kit.
CMC trigger
ATI Carbine Buffer Tube Kit
Stag Upper
Rookie Builder

How long is your lower receiver extension (buffer tube)?
How deep is it inside?

I am an idiot... when I went to measure I noticed that I put the original tube back on.
When I disassembled it after getting the second tube I sat them side-by-side to try and see any difference,I could not see any difference.
I then re-installed the original tube. I just now installed the ATI tube, and everything works perfect!
So, both tubes measured approx. 7.25" long, but the first tube had a depth of about 6.80 while the second (ATI) had a depth of 7"

glad u got it worked out

for the readers..  first flag

The spring was 12 1/4”, replaced with a new spring, 10 1/2”,

I 'believe' the 12-1/4 is for fixed stocks, not what appears to be an adjustable one on your rifle.

also... this is  a bit 'odd' in that past posts show the ATI tube as being the one out of spec with short bores.

are both tubes ATI?

The first, out of spec, tube was from a company in Utah.
I can not find my receipt, but it specifically said they were made there (in Utah) and that it was commercial specs.

The only people who have not pulled the occasional bonehead maneuver are the ones who have never done anything.