Don't use a knife on the contacts. That will work for a bit by getting to clean metal but it has also taken off the silver/
plating done to the contacts resulting in another failure further down the line.
Per a post above, it is better to use a pencil eraser for this it cleans the contact without scraping off the contact metal.
Yes, I have done this on circuit boards for a long time. Learned the trick from field service.