I have been using DT Register for several months and haven't had many issues I wasn't able to fix pretty simply. For some reason this one has me stumped. Events past December 5th don't show up on the calendar, unless I navigate to December then press the "refresh" button in the menu bar. then if I navigate away from the month and back events are missing again.
Are you using the very latest version of DT Register? If not, get updated as a first step. If you have this issue with the latest version (which no one else has reported anything similar), submit a ticket so we can check it out on your site.