I tried to find an answer with the search function but didn't find one...
Why is my activity stuck at 126 for some time now? Isn't it supposed to go up every two weeks or so?
Every 14 days (always a Tuesday) your activity points will increase by a maximum of 14 points.
You must post in that 2 week period to gain the activity points increase, post 14 times you get +14 activity points, post 3 times you get +3 activity points but it can't be more than +14 even if you post 1000 times in each 2 week period.
The next activity increase is Tuesday 22nd December.