Check this thread:
Thread 91016
---------- Post added at 11:13 PM ---------- Previous post was at 11:13 PM ----------
Also this one too:
Thread 81343
---------- Post added at 11:14 PM ---------- Previous post was at 11:13 PM ----------
TLDR:
It takes the system time for the user to be added to the group of users with PM functionality, so just be patient and keep posting!