so it's a bit difficult to be sure what is wrong. i'm working from an old kindle 3 so the process may be a bit different.put them into the documents folder of my kindle
Plug the usb cable into your pc,
turn on your kindle, plug the usb cable into the kindle,
you should get the USB Drive mode notice.
now click on windows explorer and under computer you should see Kindle,
click on kindle to open the directory.
the directory should have the following: documents, music, audio.
click on documents and see if the files you think should be there are actually there;
if the books are there confirm that the extension is "mobi".
eject your kindle from your pc.
now check your home page on your kindle and see if the books are there.
you don't say where you got the ebooks that you loaded; if it was from this forum then you can be sure that there are not drm'ed. however, if you got them from somewhere else, then try this. download mobi ebook reader to your pc and try and open the ebooks that you loaded onto the kindle from your pc copy. if they don't open then you have drm'ed copies and they will not open.
let us know how you get on. good luck.