Reason 1- You have accidentally turned off the quick access feature in your settings. Go to your file explorer and click on the View tab to check this. Then, make sure that the Quick Access option is selected. If it is not, click on it to turn it on. Reason 2 - you have too many items in your quick access folder. By default, quick access only displays the most recent 20 items. So, if you have more than 20 items in your quick access folder, the older items will be replaced by the newer ones.
You can change this by going to your file explorer and clicking on the View tab. Then, click on Options and select change folder and search options. Under the General tab, you will see an option to change the number of recent items displayed in quick access. Enter the desired number and click OK.