Thank you for the responses @floridameerkat and @katrinasforest ?
The activities show the + fame but if my sim poses for the paparazzi, a green star appears beside her head, so I know fame gain is going on. I thought it will happen with all activities that provide fame but that seems not to be the case.
Anyways, I really did notice the fame bar jumps here too. I played her a lot today and let her play the DJ in a club for another sim day or so and finally it worked! So floridameerkat is right, it takes a lot longer when not acting.
So I guess I was just too impatient ?
Thank you katrinasforest for the tip with the fame freezing cheat, I guess I will use it, so she does not drop down at least.