I tried all of the above and still couldn't remove the drm.. And then I found the problem. I discovered the kindle serial number was incorrect. The 2nd & 3rd digits were wrong, they were showing as BOO but in fact should be B00 (zero zero and NOT the letter 'O'). After I changed them from the letter 'O' to zero, Hey Presto, drm removed and all conversions worked.
If you need to change your kindle serial number heres how to do it. Click on preferences (top right) in the Calibre main window. A new box appears, click on the green plug-ins icon (a puzzle piece) under advanced. On the left hand side near the top, click the box 'Show only user installed plug-ins'. Now double click on 'File type plug-ins'. Click on the first plug-in (should be DeDRM (ver) DiapDealer blah blah blah) and it should turn green. Now down the bottom click on the button 'Customize plug-in'. A new small box appears, click on 'eInk Kindle ebooks', and you will see your kindle serial number. Click on the green button to add, new window appears, then type your kindle serial number making sure you use zero's and not the letter 'O'.
Click ok, then click ok again. At the top left of the large box click Apply, then close that box, then close Calibre and restart it, and Hey Presto all should be ok..