What would cause a piston driven system to short stroke?  I tried the usual tricks that tend to fix short stroke problems on 6.8 gas guns. I dropped in a heavier buffer, I recleaned chamber and bolt, I swapped lowers with another 6.8,  I yelled at it, I even threatened it and said I was going to get a 6.5 but the bastard still won't chamber the next round.  So whats the deal?  Heres my build details, not sure whats relevant:    20" barrel, rifle length gas port.  AA piston conversion kit with  AA low mass bolt carrier.  carbine buffer tube and spring, with heavy buffer.  I also tried running it with a lower from my other 6.8 which has the a2 rifle buffer extension tube and rifle buffer.

What gas position, do you have your block set to? Have you tried using a lighter buffer? Is your gas block properly aligned?

I had to lighten the buffer to get my 7.62x39 to cycle w/ the Adams Arms
carbine kit.  Took out the internal weight which dropped the buffer from a
standard 3 oz. down to 1 oz.  

As already mentioned the gas allignment and gas setting are the 2 biggest possibilities.    Again, if it's short stroking you first need to check your gas getting to the piston.  Pull your gas block and look at where the gas port is leaving its mark on the inside of the gas block.  After that make sure the gas is set to the highest setting and not set to the much lower setting for running a can.  If both of those are correct then try blowing through the block from the piston side to ensure there are no blockages within the gas block itself.

Also, you're typically going backwards if you try fixing short-stroking with heavier components, assuming it truly is short stroking.  The 6.8 shouldn't need the lightened carrier, that was designed for things like the 300BO where the burn is so short, or for competition where tunability is highly desired.  The 6.8 should run just fine with the standard system.  Now on the flip side one other possibility is your system is WAY too light with the light carrier, allowing it to cycle faster than the magazine can raise the next round, even with the heavier buffer.  Check your deflector, are you getting brass on it?  I don't believe it's possible to get brass on the deflector if the AR short strokes.  This could be a sign you need to slow it down further with a full-mass carrier or go up more still with the buffer weight, or go to a stiffer spring which is the cheapest fix.

I'll attest to not being an expert in 6.8SPC, but it reads like your system is crossed up. Rifle barrel & gas, but a heavy carbine buffer and spring, on lightened BCG.
Sounds like your system may be fighting against itself.

Carriers are rarely the problem, so let's hold that off to the side.

I would probably start with the buffer. I would go DOWN not up in buffer weight if it's short stroking. If you have a punch handy, you should be able to knock out the roll pin in the buffer  polymer cap and tube and slide out the weights. Try running it with only one weight or none and see if that cures the problem.

Next would be the extension. The Vltor A5 was made specially for rifle length systems to give them an adjustable stock. It's a spec piece designed to buffer rifle system, which a carbine really wasn't (not to say it can't be used.).

My 6.8 aa setup had problems with one magazine.  Switch to a Barrett magazine and worked without any problem.

Go to a full auto/full weight carrier.

So I checked the gas block and if it wasn't aligned before, it is now.  I also swapped out the low mass carrier with the standard one and replaced the heavier buffer with a standard one as well.  Hopefully that solves the problem.  I'll let y'all know.

Using their lightweight carrier and removing some of the weights in the buffer cured my short stroking problem with my 6.8.

I took all the weights out of my 6.5 buffer it cycles fine now, it would skip before. The rifle length stock and buffer is plenty heavy.