If, what you mean is that your Plus! isn't moving your logs to a new directory at the end of the month, then there's probably a mistake somewhere. Mines does it as soon as I sign in to MSN on the new calendar day...
The only thing I can think is to make sure that under Logging -> Archiving you have "Create a directory each
month for every file" selected and that both the "Instant messages" and "Events" boxes are checked.
Sorry if this isn't what your asking, just post back