I believe I may know what the problem is. I have been using ASI Air for a long time now, since it first came out.
I recently started having plate solving problems as well. I also just got the ASI AIR PRO and am also having plate solving issues.
What I discovered, is that when the camera is oriented "level on plane" (i.e. if using Alt - Az mount and image appears right side up and proper left right) plate solving fails. If I simply turn the camera about a 1/4 turn, take a picture and plate solve, it works just fine.
My guess is that the camera image "up" is somewhere around 350 to 360 degrees. If I understand plate solving correctly, what it does is try to map the image to know patterns and has to do this for all angles until it gets a match. I think it starts at 0 degrees and goes around clockwise. And being the "up" in the image is around 350-360 degrees, its just taking it too long to get there.
Thats my best guess, but its very repeatable for me.... if plate solve fails, i rotate the camera slightly and then it solves in about 3-5 seconds.