I had this same issue with playing through the DLC campaign and noticed no cheevos popped. At 1st I thought I got kicked offline. Nope, that wasn't it. I deleted all the episodes from Season 1 when I completed them. I only reinstalled E1 because it is needed for the subsequent episodes and the DLC to run. So I deleted E1 and 400 Days and reinstalled all 6 episodes and the cheevos popped as they should. I truly don't know what fixed the 'issue', so that is all I can offer as possible help.
...not suicidal, just homicidal...