Try unchecking mirroring, the rechecking it. to get the displays to merge. As well as Gather windows.
Other things to look in to is what kind of cabe/adapter, and keep in mind that even if it is new, it may not be up to spec, especially if you get them at a clearance type of store, some times that can cause issues.
I have a Belkin adapter I got from Apple, and a mid priced HDMI cable from walmart. I have never had an issue connecting to any display.
Did you try the pram reset I sent in the link? This resets all the audio and video, as well as other things. Also some tvs are just difficult to connect to. My machines all to seem to auto connect, but I sent you some work arounds, I am glad you are up and running>
Happy holidays