I have the same issue - played through the campaign on wargamer and was careful to be sure I had all the secondary's completed for each mission before moving on but no achievement. Have re-done about two thirds of it again - working backwards (!) - on easy to save time, but no achievement so far.
Really annoying that you can't check to see where the missing one is.
Erm are you sure the game let's you get the achievement when you don't play on at least medium? Because all the achievements for the campaign seem to be like that.
I have never seen it suggested anywhere before that these only unlock on medium or higher - but I have seen it claimed that easy is enough. Does anybody know for sure?
Certainly "Brigadier General" is a campaign achievement that is supposed to unlock after completing the campaign on easy (or higher). I don't know about any others as I got them all (except Hard Boiled) by playing it through first time on the hard setting.
For me it's exactly the same... I got all achievements from the campaign except hard boiled for no clear reason. I did the campaign 4 times! (easy, medium, hard and one extra for the secondary objectives).
yeah, I have the same problem, I did one playthrough on wargamer, 3 on easy and replayed the missions which have difficult objectives. I'll check if the thing on mission 19 works. I got most objectives of the missions not one at a time so i'll have a look.