I've had the same problem for about the same length of time. Just stumbled on this post looking for any info about this issue.
In the past, Twitter app push notifications have stopped because for some reason they get turned off in the app. So I would have to reactivate them. This time they're still turned on but have stopped showing up. Turning push notifications on and off has had no effect.
I'm not sure why they stopped but they have. Restarting the computer hasn't resolved the issue.
If anyone has any ideas, please pass them along.
Thanks.