Can not add customized form to related list

Can not add customized form to related list

I have customized contacts and renamed them to HD customers (Home delivery customers)

I have created a customized form called REFERENCE ITEMS via the CREATOR interface 

I then link the form to CONTACTS to create a RELATED LIST

The link stops working after a short while

When I go back to the link the system now assumes I'm linking to LEADS rather than CONTACTS

Can you please help
Thank you