Paint problem?

i recently painted a wall that had been plastered (6 weeks old) with a well known brand of matt red paint . it had 2 coats, but still looked patchy. i bought another but mistakenly got a silk version. i decided to use it anyway but it went mottled overnight. a few days later i gave it another coat but now it just looks awful . it seems to have dark patches in areas almost as if it is wet. any suggestions?