By default, Windows will hide removable media drives that don't have anything on them such as card readers. This is most likely done to avoid cluttering up your Explorer view with a bunch of empty card reader slots that aren't being used.
There is a setting you can change so that your card reader drives are always shown in Windows Explorer whether they are empty or not. To do this you need to go to your Folder Options settings. To get there open Windows Explorer or Computer and go to the Tools menu and then choose Folder Options. If you don't have the Tools menu shown then you can click on Organize and then click Layout and check Menu bar to have the menus show up in Windows Explorer.
Once you are in the Folder Options section, click on the View tab and uncheck the box that says Hide empty drives in the Computer folder. You can change other options here to such as having hidden files and folders shown as well as having Windows show file extensions for all files which is very handy. After you make your selections click on Apply to Folders to have the setting applied to all folders you open in the future. And now you should be able to see your card reader drives even when there is nothing in them.