Hi,
It seems the magnetic surface of your VHS-C tapes have been affected due to fungal growth which is usually seen after prolonged storage and non-use. Your VHS tapes will have some whitish or grayish deposits when viewed from the transparent viewing pane on the side. See attached image below:
To get rid of the fungus you'll have to get it cleaned by a professional.
Also, after cleaning the tapes, get it copied to a new cassette or a DVD which has a longer life.
Hope this solves your problem. For more help and clarifications, please reply to this post. Thanks