I do not know how do you have your buttons.cfg file set, perhaps you are missing the context on the mailbox. As I said before, mwi works in realtime if the event is sent properly. You can always start fop2_server in full debug mode to see what happens after the event is received, there are usually some lines that might give you an idea of what is wrong or going on, you can start fop2 in full debug with the command line options:
-X 511 -l /tmp
That will write a log file in /tmp (if you use version 2.26 log files are output.log and error.log, if you use 2.27 is only one log file named fop2_debug.log).
Run in full debug and check the output ..