Similar experience on my last Art Obs mission. Messages were rated as wrong location or wrong location and poor visibility. Admittedly there was cloud about...
...but as there isn't really a visually identifiable target that you can keep 'eyes on' or actual shells aimed at it, it's hard to decide when the time is right. It's my understanding that barrages in the vicinity are 'for atmosphere' and are not really directed by the player.
However, it's still my impression the parameters for successful Art Obs messages (and possibly also photos) should be significantly looser. Especially as the sim doesn't precisely mark a visible target that the player can ID and keep in view - and can't tell when the imaginary fall of shot that the message is *later* reporting was or wasn't visible to the player aircraft at the time the shells fell.
In the meantime, my lesson from the last show is to risk it and go below cloud while spotting.