If the rounds eject fine manually, its likely you have an under-gassing condition resulting in a short-stroke. When you said your single round is reloading into the chamber after firing, it is possible that your spent case is not being brought back far enough to reach the ejection port.

The first thing you need to verify is that your gas block is aligned with your gas port on the barrel. I've seen a few rifles that have the gas block clocked off-center, I've also seen a few barrels where the gas port is not at 12 o'clock, either of these will give you endless problems. Next, confirm that your piston rings are in good condition and providing a sufficient seal. To do this remove the cam pin, extend the bolt, and stand the BCG vertically on the bolt face. If the bolt retracts your piston rings aren't providing an adequate seal, there are tons of videos on YouTube about this...well there was. You should also verify that all three of them are not lined up to allow the pressure to blow-by the piston. If this checks out, head back to the range and try loading a single round in a magazine, leave the magazine inserted, and fire the round. The bolt should lock back on the bolt catch. Do this 2 or 3 times with a couple different types of ammo. If it doesn't lock back you're definitely experiencing a short stroke condition due to under-gassing. I'm assuming you probably don't have an adjustable gas block, so you could try a lighter buffer or lighter buffer spring.

Since this seems to be a wide spread issue, it is possible that with the short barrel combined with cold Canadian weather, there isn't enough pressure and 'dwell time' to sufficiently cycle the bolt. I'm not sure what gas-length these rifles are, but they should be pistol length. I highly doubt that a carbine length will be reliable with a 10.5" barrel in 7.62x39. Because x39 is a lower pressure round than 5.56, and uses much faster burning powders that produce less pressure at the gas port, you either need to make the gas port larger, or move it closer to the chamber to get adequate gas to reliably cycle the action. This is magnified with cold weather when powders burn slower and produce less pressure. Even my 10.5" 5.56 occasionally short cycles in cold weather depending on the ammunition. I have a 14.5" & 16" 7.62x39 uppers with carbine length gas systems and they will 'short stroke' with anything larger than an H buffer even in hot weather. I would be interested in knowing the gas system length and the gas port diameter, that should be a good indicator.

Best of luck, tagged for interest!