A user, loopback0, reported that PM notifications are not working consistently, particularly with flag PMs and manual PM threads. loopback0 shared examples of the inconsistency and clarified that the issue is not only with flag PMs. codinghorror pointed out a known issue where adding people to a PM after the fact can cause problems. codinghorror identified the specific issue with automatic flag handling responses not generating notifications. The issue has been acknowledged, and a fix was pushed by zogstrip with a commit to the Discourse GitHub repository: https://github.com/discourse/discourse/commit/eec1921ba9a99198d64ede4c7ddfed85505cb45b.
Initially detected on TDWTF but repro here.
Profile:
PM list:
Didn’t have a browser open here at all between flagging and coming back to check.
Have only noticed this on TDWTF since beta 4 but may have been more recent than that.
More examples which show the inconsistency here:
What is your tracking status on that PM ?
@zogstrip did you add a recent suppression here?
If it helps, it’s not just flag PMs. See the link above.
I was in a PM thread where I got no notification for the OP or the first reply, then I made the second reply, then got a notification for the third.
Note that if you add people to a PM after the fact it will not work right since that person is (incorrectly) added with a state of tracking instead of watching. This is a known issue.
Also the first post here is not clear to me. What are the exact repro steps?
For those - it was just flag a post. When @sam dealt with it and the response was sent in a PM, there was no notification.
As for repro around actual PM threads started by users manually - I’m not sure, there doesn’t seem to be any real consistency there.
Oh I see, so the automatic flag handling responses, which post standard replies to the “flag as PM” topics based on the button the mod/admin clicks to deal with the flag, are not generating notifications. I agree, something for @zogstrip to look at.
Thanks @loopback0 for reporting that issue. I just pushed a fix 
https://github.com/discourse/discourse/commit/eec1921ba9a99198d64ede4c7ddfed85505cb45b
