free/busy data disappears

Exchange 5.5, SP4 on NT4.
I’ve got a handful of people who publish their free/busy calendar data for 4 to 6 months out or more, rather than the default two months. Some of these, but not all, have reported that periodically the information seemsto disappear, leaving someone who tries to schedule with them more than 2 months in advance seeing a wide-open schedule when in reality there are appointments that are not visible. Using the /cleanfreebusy switch resolves the problem, as does the gwclient utility. But the day after I fix it, the problem reappears — none of the data is published further than 2 months ahead. Short of setting their outlook to run the /cleanfreebusy every time they open it, what can I do to fix the issue?