I've noticed that the reports tend to get hung up inside the tiki_user_reports_cache sometimes as well. I checked the database using phpMyAdmin, and the cache just gets filled with changes or updates that it wants to send out in reports but can't for some reason and just halts the reporting completely. The only way I've found to get reports to start sending again is to clear the whole cache. The reports will start sending again for a couple days and then just stop, and begin filling up the cache.
Anyone know what could be causing the reports to get hung up like this?