Is your PC allowed to go to sleep?
If so, whether by design or a failure of the program, MC doesn't seem to wake up to process a Guide Update. It will (should) try to do the update later when the PC wakes up, if it misses an update. But there were some changes in that area a while back, so something may be amiss.
I have a Windows Scheduled Task set up to wake my HTPC five minutes before the schedule Guide Load process in MC. That has been the only reliable way to get the Guide to update. This makes me very unhappy. But I have to run a Windows Scheduled Task to get my EPG anyway, since MC kills the process before it finishes occasionally, when it runs over 20 minutes. So I work around MC's shortcomings. That makes me very unhappy as well.
It is a pain in the backside that the only way to find out that the Guide hasn't been updating is to scroll the Guide to the right in the Guide to see how many days ahead you have, or to look at the "To Be Recorded" list in Standard View. It would be very nice if there was some reporting of a failed Guide Load process, probably via email.
BTW it would pay to note your MC version in new threads with issues like this, because there have been quite a few updates recently. I'm still on 20.0.33, waiting for the WDM issues to settle down.