This could be due to the fact that these items were not previously downloaded to your local computer when you were online. Outlook only stores a limited number of emails, so if these items were not accessed recently, they may not have been saved for offline use.
You can manually download all your emails and other items for offline use by going to the "Send/Receive" tab in Outlook and clicking on "Update Folder". You can also go to "File" and select "Work Offline" to ensure all items are downloaded for offline use.
Yes, you can check your Outlook settings to make sure that the items you want to access offline are selected for synchronization. Go to "File", then "Account Settings", and click on "Account Settings" again. Select your email account and click on "Change". Under "Offline Settings", make sure the option to download all items is selected.
Yes, Outlook has a feature called "Cached Exchange Mode" that allows you to access your items even with a slow or unstable internet connection. This mode stores a copy of your emails and other items on your local computer, allowing you to access them even when you're offline.
If you're still unable to open particular Outlook items when you're offline, it could be due to a corrupt Outlook data file. In this case, you may need to repair or recreate your Outlook data file. You can do this by going to "File", then "Account Settings", and clicking on "Data Files". Select your data file and click on "Open File Location". Then, you can try to repair the file or create a new one. If you're unsure of how to do this, you can contact Microsoft support for assistance.