Happens to me too sometimes, but I think there can be a number of reasons why you're still directed into the jam:
* the moment you are directed into the jam, Waze might think the actual delay is not that big or not big enough to give you a different route. Jams can grow big quite quickly sometimes.
* maybe there were not yet enough wazers in the traffic jam for Waze to really consider a traffic jam in that spot. I don't want to be rerouted because one wazer is standing still next to the road and forgot to switch off waze for example.
* traffic jam is there for a while already but when all wazers would be redirected around it, nobody will report that the traffic jam is gone so maybe Waze sends another unlucky wazer into the jam to get confirmation? (really not sure if Waze does this but sometimes it feels like this )

I get rerouted around traffic jams quite a lot, but I always see that that happens when there are quite a lot of traffic jam reports made by wazers (meaning: there are quite a lot of wazers in the traffic jam) so I think Waze needs quite a few people in the traffic jam before he starts rerouting others. When I look at most WazeWin pictures, you always see a lot of wazers in the traffic jam and nearly nobody (except the guy taking the Wazewin screenshot) on another route.