It is simply hit and miss.
I've had a flight not too long ago where the SD came up to me and said "Mr. RatherBeInYOW, there will be an agent waiting for you on the bridge with a new boarding pass" and there in fact was. I've had concierges meet and run with me to make flights. I had a concierge with me one time where a flight to YOW had just been closed and she had the GA re-open the flight, issue me a BP, close it and re-print the manifest and walk me down to the plane before closing the door. I had a concierge call me while connecting YYZ, find out where I was, and then call the gate and make sure they did not close it until I boarded. Sometimes AC does all sorts of stuff that make me happy to be SE, and I think paid J gets a lot of the same treatment. This is when it goes well.
Other times they mess up. Nobody to be found with new BPs while getting off wildly delayed flights and then massive queues (even with priority) at the airside transfer desks in YYZ. Agents in YYZ Dom refusing to help you out. Deciding to proactively rebook me on to a later flight and not tell me about it, and then I find out when I make it to the gate of the flight I was originally on in plenty of time. This happens too often as well sadly.
Your best bet is to be as proactive as you can. Check online or call AC if you have a priority number to see where you are at, and whether you have been rebooked. Don't assume that you are being fed good information from an agent on the plane, or at the gate. Sometimes you can find out more than they seem to know by simply checking online or calling and finding someone who does know what is going on.
Finally; you are not going to get explanations. You're going to get, at best, token compensation. Chalk this up to "*&$% happens" and move on.