• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
Most often it's low (or high, can't remember) literacy, or there's no valid target, such as no one of the primary culture is in the province, or any one they can assimilate to. For example, a Polish Aristocrat is not going to assimilate to a Russian Labourer.
 
Not sure how it works. I looked one time and I had british converting to yankee, dixie, and afro american. Each group, and they were in the same state, were converting to something different. Didn't know it could do that.