Hmmmm ... How to answer this without violating the terms of a NDA.
I know what at least part of the problem is with the Bolt on the manufacturing side and it has to do with the rate at which certain parts specific to that car can be built. I don't know whether the capacity constraint that I'm aware of is the only one ... it might not be. Rule of thumb in the auto manufacturing sector is that if your equipment has a problem, that's OK as long as someone else has a bigger problem, so you're not the one in the immediate line of fire, and we're not, YET ...
Lake Orion is being idled because small cars in general, and the Sonic in particular, are not selling well. (Fuel prices are too low. Likely the same reason the Bolt is not selling well in the USA.) They can not simply switch over and make all Bolts to clear the order backlog because the rate at which Bolt-specific parts would have to be manufactured to do that, is not even remotely close to being possible at the moment.
I can't answer why GM has orders going unfilled in Canada and Europe while having unsold cars in the USA. They can't just take cars already built for one market and move them to a different market. There are too many differences between them, not the least of which is the paperwork declaring which standards it was built to. It looks like they built too many for the US market while not anticipating demand elsewhere.
GM wants to sell as many of these as they can in the USA where it helps with CAFE. That may have something to do with it.
"Flexible assembly lines" are all well and good but parts specific to each vehicle still need to be stamped out and placed in fixtures and welded together, and we can't just snap our fingers and put in more fixtures and more robots to get the job done faster overnight, although we're trying ...