Tigole has announced a temporary hotfix implemented on Blackwing Lair until a more permanent solution can be created for next patch:
I thought I’d take a few minutes to clarify some issues with Blackwing Lair.
First, in regards to the Vaelanstrasz the Corrupted encounter, the way that it currently works is that once you begin the encounter (click through his gossip, watch the scene play out and aggro him) you have 1 hour to make as many attempts on him as you like, but if you fail, he plays his script, despawns and you cannot progress. This has always been the design behind this encounter and is very much intended. We will, however, be making a change in the near future to the encounter that will allow you to get a 1 hour attempt on him per day. Currently, you get a 1 hour attempt per instance reset. I’ll offer an explanation although I’m not sure it will make much sense. When the encounter went live and an issue with the door arose, we found a way to hotfix it. A hotfix is a change that we can make immediately to live realms without patching customers or bringing the servers down. However, the hotfix we were able to apply made it so that Vaelanstrasz would only be available once per instance reset. Without understanding how these events are scripted, it’s difficult to see how the two are related but they are. We already have it changed internally so that players will get 1 hour per day (rather than reset) but this change cannot go live without patching our players and restarting the servers. Hopefully, this patch will go out soon. In the meantime, it will be considered “working as intended” that the dragon is available for 1 hour per reset.
Regarding the lag that some are experiencing in Blackwing Lair, our Live Operations team is aware of these issues and analyzing them. Hopefully, we can make some fixes soon to help alleviate the lag.