Fully exit Opus and restart it, in case the problem is with the Windows change-notification system losing track of the request from Opus to listen to changes on the drive(s) in question.
Go to Preferences / Miscellaneous / Advanced and make sure no_external_change_notify is set to False (which is the default).
Check that Windows Explorer is able to see changes on the same drive, when you make changes using Directory Opus or something else that isn't Windows Explorer. If not, the drive may not be sending change notifications at all (or may be sending them incorrectly; or may be sending them, but very slowly as in the Samba case below).
If it's a network drive, make sure Preferences / File Operations / Options: Detect external file changes on network drives is turned on.
If it is a network drive on a non-Windows system then it is probably shared using Samba at the other end. Because most Unix/Linux filesystems do not support change notification, many versions of Samba implement notification by polling the drive every X seconds, where X is configurable on the server but defaults to something quite large, sometimes a couple of minutes. This means you often don't see changes made to Samba network drives for a long time, if it was something other than Opus which made the change. Very old versions of Samba do not support change notification at all.
If NTFS junctions, softlinks, mount-points, subst, etc. are involved then they are worth mentioning. Opus should detect changes through them but they can still complicate the situation and are worth keeping in mind.
If the problem only happens on a certain drive, or below a certain folder, or type of folder (e.g. a Library, or a Library pointing to a Network Drive), that may be significant and help determine where the issue is coming from.
If the issue is that some or all files do not show up at all, check that you are not filtering them out in some way. You can use Folder > Show Everything in the default toolbars (or click the hidden count on the status bar) to disable all filtering in the current folder tab.
Note that other programs may also send messages to DebugView and it is possible that some of the output you see will not be from Opus.
You don't need to understand the DebugView output, just post it to the forum along with details of the folder/path you are viewing and which changes you were making to the files below it when the output was generated.
When you are done debugging, quit DebugView and turn off the notify_debug and shellchange_debug settings in Opus.
Presumably the problem will manifest in one of two ways: Either no change is detected at all or a change is detected but for a path that is different to the path Opus thinks it is viewing.
If you don't see any changes at all, create and delete a couple of files on C:\ to make sure the debug output is appearing. Even when notification is working fine it might take a few events to trigger the debug window to appear. The best bet, once you're in a situation where notification isn't working, is to switch on the debugging mode, then create/delete some files on C:\ to make sure the debug window is visible, and only then start to make some changes in the folder that's having problems to see if any debug information is displayed for it.
If needed, post the output you see in DebugView to the forum for advice. You can use File > Save As from DebugView to save the log to a text file, or take a screenshot (be sure to make the columns wide enough to see all the information).
A tool that may come in handy when diagnosing these problems is Microsoft's Process Monitor (not to be confused with Process Explorer by the same people).
If Opus doesn't detect any changes, check which changes Process Monitor is able to see. The details it shows may help diagnose the problem. Within Process Monitor, you can save a .PML log file, to zip and submit as part of bug reports, using File > Save.
Automatically update folder content view?
Auto-refresh not working any more?!
Live view of network share?